Check Engine OBDII Diagnostic Trouble Codes (DTC)

P0300 - Random Cylinder Misfire Detected

The engine computer (ECM) constantly monitors the engine performance. The code P0300 is set when the ECM detects that multiple cylinders are misfiring at random. When a cylinder misfires, the excess of unburned fuel entering the exhaust can overheat the catalytic converter. If the ECM detects that the misfire rate is high enough to damage the catalytic converter, the Check Engine light will flash repeatedly to warn a driver. The trouble code P0300 could be caused by many reasons, for example, low fuel pressure, intake air leak, stuck open EGR valve, ignition system problems, etc. Read more »
Driving with a misfiring engine can damage the catalytic converter, so it's best to have your car checked at as soon as possible. Your mechanic will have to scan the car computer with an OBDII scan tool and perform a number of tests to find what caused the random misfiring. Read more »

P0301 - Cylinder 1 Misfire Detected

The code P030(X) is set when the engine computer (ECM) detects that a cylinder number "X" is misfiring. The code P0301 refers to the cylinder 1; the code P0302 refers to the cylinder number 2 and so on. Driving with a misfiring cylinder can damage the catalytic converter, so it's best to take care of the problem as soon as possible. Read more »

P0302 - P0312 Cylinder X Misfire Detected

See P0301.

P0325 - Knock Sensor 1 Circuit (Bank 1)

Knock Sensor

Knock Sensor

A knock sensor sends the signal to the engine computer (ECM) if the detonation or "knocking" is occurring in the cylinders. Basically the knock sensor works as a microphone. Usually a knock sensor is bolted to a cylinder block, so it can translate the vibrations from the cylinders into the electrical voltage. The diagnostic trouble code code P0325 is set when the sensor voltage is out of the expected range. The problem could be with the knock sensor itself, its wiring or some other problem. For example, a bad knock sensor was a common issue in many Nissan vehicles.
A knock sensor can be checked with a lab scope or OBDII scan tool; take your car to your mechanic for proper diagnostic.

P0340 - Camshaft Position Sensor A Circuit (Bank 1)

Camshaft Position Sensor

Camshaft Position Sensor

The camshaft position sensor (CMP) sends the signal to the engine computer (ECM) about the position of the camshaft. The ECM uses this signal to control the ignition, fuel injection and variable valve timing. The diagnostic trouble code P0340 is set when the ECM cannot detect the signal from the camshaft position sensor. This code could be caused by a bad camshaft position sensor itself, as well as problems with the wiring or connectors, engine mechanical problems, the ECM problems, etc.
The Nissan Technical Service Bulletin (TSB) EC04-018 for some Nissan Altima, Quest and Maxima with the 3.5L V6 engine, for example, lists a replacement of the camshaft position sensor(s) as the solution for the trouble codes P0340 / P0345.
The Ford TSB TSB 06-2-9 describes an issue with some Ford trucks and Mustang GT vehicles where a malfunctioning diode or open phase connection in the generator (alternator) can cause the code P0340 and/or some other codes. A bad camshaft sensor commonly causes stalling and misfiring with the code P0340 in the Chrysler PT Cruiser. Your mechanic may need to do a few tests in order to find the cause. When replacing the camshaft sensor it's always best to use an original part bought from a dealer.
The code P0340 refers to the camshaft position sensor of the Bank 1 or if the engine has only one camshaft sensor. The code P0345 - Camshaft Position Sensor A Circuit (Bank 2) refers to the camshaft position sensor of the Bank 2.

P0341 - Camshaft Position Sensor Circuit Range/Performance

The engine camshaft rotation is synchronised with the crankshaft. The engine computer (ECM) constantly receives the signal from the crankshaft position sensor (CKP) that is compared to the signal from the camshaft position sensor (CMP). The trouble code P0341 is set when the camshaft position sensor (CMP) signal is out of expected range or if it is not timed properly with the crankshaft position sensor (CKP) signal.

What can cause the code P0341:
- a faulty camshaft position sensor
- improperly installed sensor
- damaged or misaligned reluctor wheel
- foreign materials between the camshaft sensor and the reluctor wheel
- open, short or bad connection at the camshaft position sensor connector or wiring
- improper timing
- a timing belt or chain jumped a tooth
- stretched timing belt or chain
- problems with variable timing mechanism
- electrical interference from the secondary ignition system components to the CMP sensor circuit caused by high resistance in the ignition wires or spark plugs or faulty secondary ignition components.

Examples:
An Acura service bulletin describes an issue where the stuck or contaminated VTC control solenoid valve can cause the trouble code P0341. The bulletin recommends updating the ECM software and replacing the VTC solenoid valve if necessary. A service bulletin for the 2003 V6 Jaguar models mentions checking the resistance of the spark plug and replacing the spark plug along with the coil if the spark plug's resistance is too low. A technical service bulletin (TSB) for late 90's Saturn vehicles describes a problem where incorrect resistance in the ignition wires and secondary ignition system can cause the DTC P0341. A Chrysler TSB for 2010 Ram pickup trucks equipped with a Cummins diesel engine recommends re-flashing the ECM for the P0341 and some other trouble codes. In some 4-cylinder 2003-2010 Honda Accord, Civic, CR-V and Element engines, a stretched timing chain can cause the Check Engine light (MIL) to come on with code P0341.

What needs to be checked:
The camshaft position sensor connector and the wiring must be checked for damage, poor connection or corrosion. If it's a three-wire sensor, check for 5V reference voltage and the ground at the sensor connector with the ignition ON. The proper camshaft timing must be verified; if a timing belt or chain jumped a tooth, it can cause this code. A stretched timing chain can also cause the code P0341. Symptoms of a stretched timing chain include rattling noise, lack of power on acceleration, hard starting. There are different way to check if the timing chain is stretched: for example, Honda has a special tool; in some other cars, the signal from the camshaft position sensor must be compared on the oscilloscope to the crankshaft position sensor signal. If the tune-up hasn't been done in a long time, it's a good way to start, as the high resistance in the spark plugs and ignition wires can cause electrical interference from ignition secondary circuits that can cause spikes in the camshaft sensor signal. The routing of the camshaft position sensor wiring needs to checked, as If the camshaft sensor signal wires are routed too close to the secondary ignition components, there also could be an electrical interference. The proper operation of the variable valve timing system also must be checked.

How the camshaft position sensor works:
The camshaft position sensor (CMP) monitors the position of the camshaft or camshafts. In an OHV (pushrod) engine, the CMP sensor is installed into the cylinder block. In a modern DOHC engine, one or two camshaft position sensors are installed at the cylinder head.

Camshaft position sensor (CMP)

Hall-effect camshaft
position sensor (CMP)

A CMP sensor could be a two-wire pick-up coil type or three-wire Hall effect sensor. A pick-up coil sensor generates an electric signal, while the Hall effect camshaft sensor needs a 5V reference voltage. Modern OBDII cars mostly use a three-wire Hall effect camshaft position sensors. The engine computer (ECM) uses the signal from the camshaft position sensor to know which cylinder is in the compression stroke, as well as to control the ignition timing, fuel injection and the variable valve timing system (if equipped).

Q: 2001 1.8L Volkswagen Passat turbo. Some gaskets and seals and the timing belt has been replaced, but now I got the code P0341 and the engine is hard to start. What could be the problem?

A: Double check the timing between the crankshaft and the driven camshaft at the timing belt, as well as the timing between two camshafts, as the chain that runs between the camshafts can easily jump a tooth and this will be enough to set the P0341 code.

Q: 1999 Chevrolet Malibu 3.1L V6 misfires and runs poorly. The parts store scanned the computer and there was a code P0341. Is it an expensive to repair problem? Where do I start?

A: Starting with a tune-up will be a good idea; as mentioned above, in these GM V6 engines the high resistance in the ignition components can cause arcing that could in turn cause the electrical interference with the camshaft sensor signal. If the spark plugs and wires haven't been replaced for a long time, this could easily cause this code. Check the sensor's wiring and the connector for corrosion or loose connection. Also, it's worth to check the ground cable connection as this also can cause electrical problems.

P0345 - Camshaft Position Sensor A Circuit (Bank 2)

See P0340.

P0400 - Exhaust Gas Recirculation (EGR) Flow

Ford EGR valve

EGR valve

The EGR (Exhaust Gas Recirculation) system is designed to reduce the amount of the nitrogen oxides (NOx) in the exhaust emissions. The nitrogen oxides are formed at very high combustion temperatures. The EGR system diverts a small portion of the exhaust gases back into the intake manifold to reduce the combustion temperature, which in turn lowers the amount of NOx in the exhaust gases. The EGR flow is controlled by the engine computer (ECM) which opens or closes the EGR valve depending on the operating conditions. Typically, the EGR valve is closed at idle or at wide open throttle (WOT); the EGR flow is at its maximum during steady cruising. The diagnostic trouble code P0400 is set when the ECM detects that the EGR flow is out of expected range... read full article »

P0401 - Exhaust Gas Recirculation (EGR) Insufficient Flow

Ford DPFE sensor

Ford DPFE sensor

The main purpose of the vehicle EGR (Exhaust Gas Recirculation) system is to reduce the amount of the nitrogen oxides (NOx) in the exhaust emission. The nitrogen oxides (NOx) are formed at very high temperatures. The EGR system diverts a small portion of the exhaust gases back into the intake manifold where it dilutes the air/fuel mixture reducing the combustion temperature as a result. The flow of the exhaust gases back into the intake manifold (EGR flow) is controlled by the EGR valve.
The OBD code P0401 is set when the EGR flow... read full article ».

P0402 - Exhaust Gas Recirculation flow Excessive Detected

The code P0402 sets when the engine computer (ECM) detects excessive EGR system flow. The Exhaust Gas Recirculation (EGR) system routes a small percentage of the exhaust gases from the exhaust manifold back into the intake manifold to reduce the combustion temperature. This is necessary to lower the nitrogen oxides (NOx) content in the exhaust gases, as the nitrogen oxides are formed at very high combustion temperatures.

EGR system diagram

EGR system diagram

The EGR system connects the exhaust manifold to the intake manifold through the pipes or passages, with the EGR valve separating the exhaust manifold from the intake manifold and controlling the EGR flow (see the diagram). The EGR valve should be closed when the engine is cold, at idle or under heavy load. The EGR valve opens during other modes to allow the EGR flow. The ECM monitors the EGR flow and periodically tests the EGR system as a part of on-board diagnostic OBD II or EOBD. The code P0402 means that excessive EGR system flow is detected when it's not expected. The code P0402 could be accompanied by rough or no idle, hesitation and stumbling.

What could cause the code P0402
- stuck open EGR valve
- dirty or clogged air filter
- intake leaks
- problems with a turbocharger
- incorrect vacuum hose connection
- bad or damaged DPFE sensor (Ford, Mazda)
- faulty EGRC-BPT valve (Nissan)
- faulty EGR temperature sensor
- inferior quality aftermarket parts used in previous repairs (e.g. DPFE sensor)
- damaged or missing EGR valve gasket
- issues with the EGR vacuum control
- problem with the EGR vacuum switching valve
- electrical problems with the EGR temperature sensor circuit
- electrical problem with the EGR valve control circuit
- clogged catalytic converter
- bad mass airflow (MAF) sensor
- engine computer problems

Examples:
A Ford TSB for early 00's Taurus, Ranger, Explorer, F150 and some other models recommends replacing the DPFE valve if the codes P0401, P0402 are present. A similar bulletin was issued for 2001-2002 Mazda Tribute and B4000 that also recommended replacing the DPFE sensor. A Chrysler TSB for 2007 Dodge Nitro recommends reprogramming the ECM as a solution for the code P0402 EGR Excessive Flow. Another TSB the Ram Pickup with a 6.7L Cummins Diesel engine also recommends re-flashing the ECM if erroneous code P0402 or some others are set. In some Volkswagen cars with a TDI Diesel engine, the code P0402 could be caused by a bad mass airflow sensor (MAF).

What needs to be checked with the code P0402:
If there are other codes or driveability symptoms present they should be looked at first, as the code P0402 can be set as a result of bad MAF sensor, plugged catalytic converter, etc. If the air filter is very dirty, it make sense to replace it as in some engines a dirty air filter can cause the code P0402. The EGR valve should be checked, as it could be stuck open. If the EGR valve is stuck open, the engine would run rough at idle. Sometimes a small piece of carbon can get stuck between the valve and the seat. If this is found the case, the whole EGR system should be cleaned to prevent reoccurrence. In Ford vehicles, the DPFE sensor and the hoses connected to the DPFE sensor should be checked. The DPFE sensor voltage should be tested according to the chart. If no other obvious problems found and it's an aftermarket DPFE sensor, it's worth to try if the OEM sensor solves the problem. If the code P0402 comes after some repairs, the previously work done must be re-checked for vacuum hose routing, disconnected hoses, missing gaskets, etc.

Q: 2002 Ford Ranger. I repeatedly get the code P0402, even though the truck is running fine. I checked the EGR valve and it's not stuck open. The EGR passages are clean. If I open the EGR valve manually when the truck is running the idle becomes rough. The DPFE sensor was replaced before. What am I overlooking?

A: I wonder if the DPFE sensor is an aftermarket part, because if it is, you may want to give it a try and replaced with OEM DPFE sensor.

P0403 - Exhaust Gas Recirculation Control Circuit

Electric EGR Valve

Electric EGR Valve

The Exhaust Gas Recirculation (EGR) system diverts a portion of the exhaust gases back into the intake manifold to reduce the combustion temperature, which in turn lowers the amount of nitrogen oxides (NOx) in the exhaust gases.
The engine computer (ECM) controls the EGR flow by opening and closing the EGR valve. Some cars have a vacuum-operated EGR valve, others have an electrically controlled valve (in the photo). The engine computer (ECM) constantly monitors the operation of the EGR valve. The trouble code P0403 is set when the ECM detects a problem with the electric circuit controlling the EGR valve.
This could be caused by a problem with the electric EGR valve or solenoid itself, poor connection at one of the connectors or a short or open in the wiring harness. An internal problem with the engine computer (ECM) can also cause the code P0403.
The diagnostic process is usually straightforward: the EGR valve control electrical circuit needs to be checked between the EGR valve and the ECM. One other thing that is often get missed is the ECM ground. Sometimes a bad ground can cause all kinds of electrical glitches. A friend of mine had Dodge Caravan that kept throwing bunch of trouble codes because of one bad ground connection. Sometimes the ECM may need to be reprogrammed. A service bulletin for some Dodge trucks listed reprogramming the ECM as a solution for EGR-related codes. Another bulletin called for EGR valve replacement.
Read more about the Exhaust Gas Recirculation (EGR) system.

P0410 - Secondary Air Injection (AIR) System

A secondary air injection system injects extra outside air into the exhaust during a cold start to help the catalytic converter work more efficiently. The code P0410 sets if the ECM detects a problem with the secondary air flow. Often this could be caused by a problem with the secondary air pump itself or with one of the hoses or valves that control the secondary air flow. For example, some GM vehicles had problems with water getting into the secondary air pump causing it to freeze or corrode or with bad vacuum check valves controlling the secondary air flow. Typically a secondary air pump can be activated with a proper scan tool, so your mechanic should be able to find a problem fairly easy.

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

Catalytic converter

Catalytic converter

The catalytic converter is installed in the exhaust and is an important part of the vehicle emission control system. To monitor how well the catalytic converter does its job, there are two oxygen sensors installed in the exhaust: one before and one after the catalytic converter. The engine computer constantly compares the signals from both sensors. If the catalytic converter can no longer do its job, the engine computer turns the Check Engine light and sets the diagnostic trouble code P0420 for Bank1 or P0430 for Bank 2.
There could be a number of reasons for the code P0420, but in most cases, it's the problem with the catalytic converter itself. A failed catalytic converter will have to be replaced. A catalytic converter is quite expensive part, but there is a chance that it might be covered by the emission warranty. In US, an original catalytic converter is typically covered for 8 years or 80,000 miles. Sometimes car manufacturers extend the emission warranty, so if you have the trouble P0420, first thing to do is to check your warranty booklet or call your local dealer to find out if the catalytic converter is still covered. If it is covered, your dealer might replaced it under warranty.
If you are planning to install an aftermarket catalytic converter, be aware that not all aftermarket catalytic converters offer the same quality. There are plenty of reports where aftermarket catalytic converters cause problems. Make sure you understand the warranty that comes with an aftermarket catalytic converter.
The code P0430 - Catalyst System Efficiency Below Threshold (Bank 2) has the same meaning only referring to the catalytic converter of the Bank 2.

Q: 2005 Mazda 3, code P0420, replaced two O2 sensors, catalytic converter, the code P0420 comes back.

A: The code P0420 means Catalyst System Efficiency Below Threshold, or in other words, the catalytic converter doesn't work properly. Most of the times the code P0420 is caused by a bad catalytic converter itself. If your mechanic has a good scan tool with a data monitor, it's easy to see comparing the signals from the front and rear O2 sensors. One thing to be aware of, sometimes the aftermarket catalytic converter may not be compatible, and the code can come back. In this case, it's best to use an original (OEM) part. The same goes for O2 sensors: it's best to use original parts to avoid issues. Original parts are more expensive, but they are manufactured to work in your car properly.

P0430 - Catalyst System Efficiency Below Threshold (Bank 2)

See the fault code P0420

P0442 - Evaporative Emission System Leak Detected (Small Leak)

The Evaporative System (EVAP) prevents the fuel vapors from the fuel tank from escaping into the atmosphere. The EVAP system draws the fuel vapors from the fuel tank and temporarily stores them in the charcoal canister. When the engine is running and other conditions allow, the fuel vapors are purged from the charcoal canister into the engine air intake to be burned. The EVAP system is sealed and the engine computer (ECM) can detect if there is even a small leak. The fault code P0442 means that the ECM has detected a small leak. A gas cap is the first thing to check in this case. Sometimes a gas cap may look OK, but when tested on a special tester it could show a leak. The problem could also be caused by a small crack in a one of the EVAP system tubes or in the charcoal canister, a partially open canister vent valve, loose tube/line connections in the EVAP system, loose seals, problems with the purge valve, problems with the fuel tank, problems with electrical components of the EVAP system, etc. For example, the Mazda TSB (Technical Service Bulletin) 01-052/06 for 2004-2006 Mazda 3 and Mazda 6 recommends checking the purge solenoid valve for intermittent sealing issues.
It's very difficult to find a small leak in the EVAP system without a special equipment. Sometimes, even with the special testing equipment (usually called "smoke machine"), it could take a lot of time for a mechanic to find a small EVAP system leak. If your car has the code P0442, take your vehicle to a reputable garage or a dealer for proper diagnostic.

Q: Code P0442 on Saturn SL2?

A: Check the fuel cap, make sure it's tight. If you think that it wasn't closed properly, the check engine light should go off by itself in a few days of driving. If the check engine light won't go off, there could be a number of other reasons, such as leaking ventilation valve, cracked charcoal canister, cracked or pinched hose, leaking seal, etc.
If the leak is very small, it could be very hard to find. Take your car to a dealer or well-equipped garage that can test your evaporative system with the special equipment.

Q: I have a 2005 chevy equinox with code P0442 what could be the problem ?

A: The code P0442 means 'Small EVAP system leak'. The EVAP system is the one that doesn't allow the vapors from the tank to escape into the atmosphere; read more above. Checking the fuel cap is the first thing to do. If the rubber seal at the fuel cap looks damaged or there is some corrosion that may prevent the cap from sealing properly, it might be a good idea to put a new cap, which is not expensive. If it was a bad fuel cap that caused the code P0442, the code should clear itself up after a couple days of driving. If the code is still there, there might be some other sources of the EVAP leak. It could be a crack or damage at the charcoal canister, or a small tear at some rubber line, bad vent control valve and few other things. If the cap doesn't fix the problem, take it to a repair shop, they can check the EVAP system with the 'smoke machine' for leaks.

P0446 - Evaporative Emission Control System Vent Control Circuit Malfunction

The code P0446 is set when the engine computer recognizes a fault or restriction at the vent control part of the Evaporative emission control system (EVAP). The description of the fault varies between different car manufacturers, but one thing is common, the code P0446 doesn't directly point to a defective part; some testing might be required to pinpoint the faulty component. Read more »

P0449 - Evaporative Emission Control System Vent Valve/Solenoid Circuit Malfunction

Charcoal Canister

Charcoal Canister

The Evaporative System (EVAP) prevents the fuel vapors from the fuel tank from escaping into the atmosphere. The EVAP system traps the fuel vapors and temporarily stores them in the charcoal canister. When the engine is running and other conditions allow, the fuel vapors are drawn by the engine vacuum from the EVAP charcoal canister and into the engine air intake. To balance the air pressure in the fuel tank, some air is allowed into the charcoal canister when needed.
The engine computer periodically runs the self-test to see if the Evaporative system is sealed tight. For the time of the test, the engine computer closes the vent valve. The trouble code P0449 means that the engine computer has detected a malfunction in the electric circuit that controls the vent valve/solenoid.
It could be a problem with the vent valve/solenoid itself or with the wiring that runs from the engine computer to the vent valve/solenoid. Sometimes the wires that run to the vent valve/solenoid get rubbed through or shorted. A bad connection at the vent valve/solenoid connector, a problem with the power supply or ground or an issue with the engine computer (ECM) itself can also cause the code P0449.
Typically the vent valve/solenoid is located somewhere closer to the fuel tank, not too far from the charcoal canister at the back of the vehicle. Many newer cars have the vent valve attached to the charcoal canister. The diagnostic procedure for the code P0449 usually involves operating the vent valve solenoid with a scan tool and testing the power supply voltage and the ground at the vent valve/solenoid, as well as checking its resistance.
Often a number of other EVAP system codes (e.g. P0452, P0455) could also be present, because if the vent valve/solenoid doesn't work properly the whole EVAP system fails the self test.
The code P0449 is common in some GM trucks (e.g. Tahoe, Silverado, Suburban) and is often caused by a bad vent solenoid itself. The vent solenoid is an inexpensive part and is fairly easy to replace.

P0455 - Evaporative Emission System Leak Detected (Large Leak/No Flow)

The Evaporative System (EVAP) prevents the fuel vapors from the fuel tank from escaping into the atmosphere. The EVAP system traps the fuel vapors and temporarily stores them in the charcoal canister. When the engine is running and other conditions allow, the fuel vapors are purged from the charcoal canister into the engine air intake to be burned. The purge flow is controlled and monitored by the engine computer (ECM). The Evaporative System is sealed and the ECM can detect if there is a leak. The diagnostic trouble code P0455 - Evaporative Emission System Leak Detected (Large Leak/No Flow) means that the ECM has detected a large leak or no purge flow. A gas cap is the first thing to check in this case. The problem could also be caused by a cracked or disconnected tube, damaged charcoal canister, blockage in the fuel vapor lines or hoses, bad purge valve, stuck open canister vent valve, etc. The Evaporative system problems are difficult to diagnose without special equipment. If your car has the code P0455 and the gas cap looks OK, take your vehicle to a reputable garage or a dealer for proper diagnostic.

Q: I have Nissan Rogue and my Check Engine light is on. I had it scanned at the auto parts store and the code is P0455. What could it be?

A: The code P0455 means an EVAP system leak detected. First thing to check, is the gas cap. If it's loose, close it tight, the Check Engine light will clear itself after a day or two of driving. A common problem in this vehicle is a bad vent control valve. It's installed at the charcoal canister at the back of the car, close to the right rear wheel. Your mechanic can check it easily, it's just a solenoid valve that closes when the voltage is applied. If the vent control is bad, make sure to use a new updated part from Nissan, it's not very expensive and is easy to install.

P0505 - Idle Control System Malfunction

IAC Solenoid

IAC Solenoid

The code P0505 - Idle Control System Malfunction means that the engine computer cannot regulate the idle speed properly, or the actual idle speed varies from the target idle speed by a certain amount. The accompanied symptoms may include difficulty starting, hesitation, stalling at idle, fluctuating idle speed, as well as too high or too low idle speed.
There could be plenty of reasons for the code P0505 to set: vacuum leaks past the throttle, clogged idle air passages, bad or clogged idle air control valve, dirty throttle body, faulty electronic throttle, problems with the wiring or connectors, ECM problems, etc. Sometimes the carbon deposits in the idle air control valve can cause it to be stuck open or closed. Sometimes even low coolant level can cause the idle fluctuation and the code P0505. Problems with the EGR and EVAP systems also can cause the code P0505. If the idle air control valve or the throttle body is dirty, sometimes cleaning the either one can solve the problem. However, in many vehicles the idle re-learn procedure must be performed after the repair is finished. In some cars, the idle re-learn procedure is performed with a scan tool, in others the ECM re-learns the idle speed after the engine is run for some time.

How idle control works

Electronic Throttle cutaway

Ford Electronic throttle
cutaway display

In modern cars, the engine computer (ECM) constantly adjusts the idle speed according to the conditions. It's done by increasing or decreasing the air flow bypassing the engine throttle. You can notice the idle speed adjustment, for example, when you start the engine cold; the engine computer (ECM) increases the idle speed to help warm up the engine faster.
In some cars there is a separate idle air control (IAC) valve or solenoid (top photo) that controls the air flow at idle. In other cars the electronic throttle is used for the same purpose.
In a car with a separate idle air control valve, the throttle is closed at idle, so the only air supplied to the engine bypasses the closed throttle through the idle air passages and the idle air control valve, which is usually installed at the throttle body. The engine computer (ECM) adjusts the engine idle speed by slightly opening or closing the idle air control (IAC) valve.
If cars with electronic throttle idle control, the engine computer slightly opens or closes the electronic throttle to add or reduce the idle air flow.

Examples:

- The diagnostic procedure for the code P0505 usually includes inspecting the idle air control system components, checking the IAC valve power supply voltage, inspecting electrical connectors and wiring, as well as testing the idle air control system with a scan tool. If beside the P0505 there are other trouble codes presented, they need to be looked at first.
- A Honda Technical Service Bulletin (TSB) dated August 2008 describes a problem with some early 00's Odyssey and Pilot models where the stuck open idle air control (IAC) valve or broken power steering pressure switch wire can cause the idle to fluctuate, with the code P0505. As a solution Honda recommends replacing the IAC valve or (and) repair the power steering pressure switch wire located right above the steering rack boot on the passenger side of the vehicle.
- A Ford technical service bulletin for the 2006-2007 Explorer, 2004-2006 F150, as well as some other models describes a problem with hesitation, difficulty starting and low idle with the codes P0505 and/or P0506. Ford recommended reprogramming the ECM and replacing the electronic throttle body (ETB) if it fails the test as a solution.
- Another Ford TSB for the 2005-2007 Ford Five Hundred, Freestyle mentions spray cleaning the throttle body as a solution for RPM fluctuation with the codes P0505, P0506.
In some early 00' Nissan and Infiniti cars with the code P0505 a bad IAC valve could even cause the engine computer to go bad as well.