Check Engine OBDII Diagnostic Trouble Codes (DTC)

Here are some of the frequent check engine OBDII fault codes or DTCs (Diagnostic Trouble Code) and their generic meaning. This is of course very basic information to give you an idea, as the interpretation of the check engine diagnostic trouble codes varies slightly between different car manufacturers. To properly diagnose a check engine light problem, take your car to a local repair shop or a dealer, where a mechanic can do some tests with a scan tool and check a number of components to determine what exactly caused the problem. Checking the freeze frame is also important. A freeze frame is a snapshot of the powertrain parameters at the moment when the engine computer detected a fault and the trouble code was set.
Often there could be not one, but a few trouble codes stored in the engine computer (PCM). In this case, a repair manual usually lists which code should be troubleshooted first, as additional codes may be set as a result of the same problem.

P0100 - Mass Air Flow Circuit Malfunction

The fault code P0100 is set if the engine computer detects an improper signal from the mass air flow sensor (MAF). It could be the sensor wiring or a connector problem, but often it's the mass airflow sensor itself that goes bad. For example, a Nissan Technical Service Bulletin for the code P0100 In Nissan Maxima says that this code might be caused by "airflow sensor that has been damaged by air-ingested dust/dirt". Read more about Mass Air Flow sensor.

P0101 - Mass Air Flow Sensor Circuit Range/Performance

A mass air flow sensor (MAF) measures the amount of air entering the engine. An engine computer compares the actual signal from the mass air flow sensor to what is expected based on the readings from other sensors. The code P0101 is set when the signal from the mass air flow sensor is out of the expected range. A number of problems can cause this code, for example, a bad mass air flow sensor, a vacuum leak, a leak in the air duct between the air flow sensor and the engine, a sensor wiring problem, etc. Even a plugged catalytic converter can cause this code. Read more about Mass Air Flow sensor.

P0102 - Mass Air Flow Sensor Circuit Low

Mass Air Flow Sensor (MAF) Similarly to the previous trouble code, the actual signal from the mass air flow sensor (MAF) is compared by the engine computer to what it is expected to be based on the readings from other sensors. The code P0102 is set if the signal from the mass air flow sensor is lower than expected. Often this code could be caused by a bad or contaminated mass air flow sensor, although there could be plenty of other reasons. Mass air flow problems are very common in many cars. For example, the code P0102 caused by a bad mass air flow sensor was a fairly common problem in 1999-2005 Volkswagen Jetta. A mass air flow sensor can be checked with an OBDII scan tool; take your car to your mechanic for proper diagnostic. Read more about Mass Air Flow sensor.

P0116 - Engine Coolant Temperature Sensor Range/Performance

Engine Temperature Sensor (ECT)

The code P0116 means that the engine temperature is out of expected range. For example, when the car is started cold, the engine computer compares the data from the engine coolant temperature sensor (ECT) and the intake air temperature sensor (IAT). Normally, when the engine is just started cold, its temperature should be close to the temperature of the outside air. If the difference between the data from the engine coolant temperature sensor and the intake air temperature sensor is too great, the computer sets the code P0116. The same code could be set if the engine coolant temperature is higher than expected.
The code P0116 could be caused by a bad engine temperature sensor itself, poor connection at the sensor, wiring problem or the problem with the coolant system. For instance, on some Volkswagen cars the code P0116 was commonly caused by a bad temperature sensor. Checking the freeze frame data should help to diagnose the problem, as the freeze frame will show the engine temperature sensor reading as well as data from other sensors at the moment the fault was detected. Take your car to a repair shop for proper diagnostic, your mechanic can scan the car computer and check the freeze frame. Read more about the freeze frame here: Why the "Check Engine" light comes on.

P0128 - Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

The code P0128 essentially indicates that the specified engine temperature was not achieved in a certain time. Most often this code indicates a bad or stuck open thermostat. Read more about a thermostat here: Thermostat - illustrated glossary.
A bad coolant temperature sensor (ECT), low coolant level or some other problem of the coolant system can also cause the trouble code P0128 to be set. Sometimes, a car manufacturer may recommend an engine computer to be reprogrammed to address this problem.

P0130 - O2 Sensor Circuit (Bank 1 Sensor 1)

Front Oxygen Sensor The fault code P0130 is set when the voltage from the front O2 sensor is out of range. This code could be caused by an open or short in the oxygen sensor circuit, bad oxygen sensor, too lean or too rich air/fuel ratios, intake leaks, exhaust leaks etc. An oxygen sensor can be checked with a lab scope or OBDII scan tool; take your car to your mechanic for proper diagnostic. Some car manufacturers may also recommend reprogramming of the engine computer (PCM). For example, the Subaru Service Bulletin WWF-89 dated 04/2002 lists replacing the front oxygen (A/F) sensor and reprogramming the ECM as a solution for codes P0031, P0130 or P1133. The code P0150 - O2 Sensor Circuit (Bank 2 Sensor 1) has the same meaning, but it relates to a front oxygen sensor of the bank 2. Read more about an oxygen sensor here: Oxygen sensor - illustrated glossary.

P0132 - O2 Circuit High Voltage (Bank 1, Sensor 1)

Normally the front oxygen sensor produces a small signal voltage (0.2V to 0.9V). The trouble code P0132 is set when the signal voltage is too high. This may happens, for example, when the oxygen signal wire gets shorted to a power wire somewhere in the harness. Too rich air/fuel mixture, a bad ground or moisture on the connectors or inside the wire harness also can cause the same problem. Sometimes the oxygen sensor itself can go bad causing the code P0132. Also, on some cars, using aftermaket oxygen sensors instead of original is known to cause the fault code P0132. Some car manufacturers may also recommend reprogramming of the engine computer (PCM) if the code P0132 is present. An oxygen sensor can be checked with a lab scope or OBDII scan tool; take your car to your mechanic for proper diagnostic. Read more about an oxygen sensor here: Oxygen sensor - illustrated glossary.

P0133 - O2 Circuit Slow Response (Bank 1, Sensor 1)

In general, this code means that on a warmed up engine, the signal from the front oxygen sensor of the bank 1 switches between lean and rich air/fuel conditions too slow or its amplitude (voltage) is out of expected range. This code could be caused by a bad or contaminated O2 sensor itself, sensor wiring problems or plenty of other reasons, such as exhaust leaks, too lean or too rich condition, intake leaks, a bad mass air flow sensor, etc. An oxygen sensor can be checked with a lab scope or OBDII scan tool; take your car to your mechanic for proper diagnostic. Read more about an oxygen sensor here: Oxygen sensor - illustrated glossary.
The code P0153 - O2 Circuit Slow Response (Bank 2, Sensor 1) has the same meaning, but refers too the front oxygen sensor of the bank 2.

P0134 - Oxygen (A/F) Sensor No Activity Detected (Bank 1 Sensor 1)

Front Oxygen Sensor

Air/Fuel ratio sensor

The code P0134 - Oxygen (A/F) Sensor No Activity Detected (Bank 1 Sensor 1) means that the engine computer (PCM) has detected that the front oxygen or air/fuel ratio sensor doesn't work as it should; or in technical language, the signal from the front oxygen sensor or the air/fuel (A/F) ratio sensor does not change for a certain time when the necessary conditions (e.g. the engine temperature, rpm, etc.) are met.
The front or upstream oxygen sensor is located in the exhaust manifold or in the exhaust downpipe before the catalytic converter. The front oxygen sensor monitors the amount of the oxygen in the exhaust gases and provides the feedback signal to the engine computer (PCM) whether the air/fuel mixture entering the engine is lean (too little fuel and too much air) or rich (too much fuel and too little air). Read more about an oxygen sensor here: Oxygen sensor - illustrated glossary.
Some cars have an air/fuel ratio sensor (in the lower photo) installed instead of the oxygen sensor. An air/fuel ratio sensor works differently, but serves the same purpose. The code P0134 refers to whichever sensor is installed.
The code P0134 is often caused by a bad front oxygen or air/fuel ratio sensor itself. For example, a friend of mine had the code P0134 on his Honda CR-V and the new original Honda air/fuel ratio sensor fixed the problem. As far as I know, some other Honda vehicles also had problems with the A/F sensor causing the code P0134.
Besides a bad front oxygen or air/fuel ratio sensor, the code P0134 could be also caused by problems with the sensor's heater or wiring, exhaust leaks, vacuum leaks, lean air/fuel mixture and a number of other possible reasons. For example on some Volkswagen cars, a bad mass air flow sensor could cause a lean condition that would in turn cause the code P0134 along with some other fault codes.
A service bulletin for 2006–08 Civic Si describes a problem where the grease would drip onto the air/fuel (A/F) ratio sensor connector causing the code P0134 or some other codes. Honda lists replacing the A/F sensor and installing a subharness kit as a solution.

P0135 - O2 Sensor Heater Circuit (Bank 1 Sensor 1)

An oxygen sensor or O2 sensor measures the amount of oxygen in the vehicle exhaust. Read more about an oxygen sensor and what does it mean "Bank 1 Sensor 1" here: Oxygen sensor - illustrated glossary.
Modern cars have a heating element installed inside of the oxygen sensors. A heating element helps the oxygen sensor to reach the operating temperature faster. A heating element is controlled by the engine computer. If the computer detects an open, short, or an excessive current draw in the heating element circuit of the front oxygen sensor of the bank 1 (Bank 1 Sensor 1), it sets the code P0135.
The problem could be with the sensor itself, or with the sensor wiring or connectors. A low battery voltage or a problem with engine computer can also cause this code.
Codes:
P0141 - O2 Sensor Heater Circuit (Bank 1 Sensor 2),
P0155 - O2 Sensor Heater Circuit (Bank 2 Sensor 1) and
P0161 - O2 Sensor Heater Circuit (Bank 2 Sensor 2) work the same way, just for different oxygen sensors.

P0136 - O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)

Rear Oxygen Sensor A rear oxygen sensor (Sensor 2) is installed in the exhaust after the catalytic converter. The term Bank 1 typically refers to the bank containing cylinder 1. Read more about an oxygen sensor and what means "Bank 1 Sensor 2" here: Oxygen sensor - illustrated glossary. The engine computer (PCM) uses the signal from the rear O2 sensor to monitor the catalytic converter efficiency. Normally the rear oxygen sensor's voltage changes slightly when the fully warmed-up engine is accelerated (rich mixture) or decelerated (lean mixture). If the voltage from the rear O2 sensor doesn't change as expected for a certain period of time, the code P0136 is set. This could be caused by a faulty oxygen sensor itself, corrosion in the sensor connector, problem with the sensor wiring, exhaust leak or a number of other causes. Sometimes a too lean or too rich condition can trigger the code P0136. In some cases, a problem with the front O2 sensor or air/fuel sensor or the catalytic converter can also cause the code P0136. The code P0156 O2 Sensor Circuit Malfunction (Bank 2 Sensor 2) has the same meaning, but it relates to the rear oxygen sensor of the bank 2.

P0141 - O2 Heater Circuit (Bank 1, Sensor 2)

The code P0141 woks the same way as P0135, but it's related to the rear O2 sensor (Sensor 2) of the Bank 1. Again, it could be the oxygen sensor itself, the wiring problem, low battery, and many other things.
For example, A technical service bulletin for 1996-99 Honda Civic mentioned the problem where the engine wiring harness is rubbing on the intake manifold bracket in the back of the engine causing the code P0141 along with some other codes and blown fuse.
A while ago my 2003 Honda Accord threw the check engine light with the code P0141, I just replaced the rear O2 sensor, cleared the code and the problem was fixed.
Read more about Oxygen Sensor.

P0153 - O2 Circuit Slow Response (Bank 2, Sensor 1)

See the code P0133.

P0155 - O2 Sensor Heater Circuit (Bank 2 Sensor 1)

The code P0141 works the same way as the fault code P0135, but it's related to the front O2 sensor (Sensor 1) of the Bank 2.

P0156 - O2 Sensor Circuit Malfunction (Bank 2 Sensor 2)

See the code P0136.

P0161 - O2 Sensor Heater Circuit (Bank 2 Sensor 2)

The code P0161 has the same meaning as the trouble code P0135, only for the rear O2 sensor (Sensor 2) of the Bank 2.

P0170 Fuel Trim Malfunction (Bank 1)

The code P0170 means that the air-fuel mixture coming in to the engine is too lean, or too rich. Usually it's the lean condition that causes this code. The code P0173 means the same, only for the Bank 2. Some car manufacturers list the description for the code P0170 as self adaptation out of limits.
The code P0170 could be caused by a number of reasons, such as vacuum leaks, bad PCV valve, or other parts of crankcase ventilation system, low fuel pressure, bad oxygen sensor, etc.
The code P0170 Fuel Trim Malfunction (Bank 1) is common in European models. Often a car may exhibit stalling, hesitation and rough idle.
In BMW vehicles vacuum leaks from cracked rubber intake air boot is a common occurrence. The intake air boot connects the air filter box with the intake manifold. Problem with positive crankcase ventilation components also can cause vacuum leaks. The mass airflow sensor problems are also not uncommon.
In some Mercedes-Benz vehicles besides other reasons, a bad mass airflow sensor can cause the code P0170 Fuel Trim Malfunction (Bank 1), as well as P0173 Fuel Trim Malfunction (Bank 2). I even had the code P0170 on my 1997 Mercedes-Benz E320. The new mass airflow sensor fixed the problem in that case.
To find the exact reason in your car, your mechanic may need to check your vehicle for vacuum leaks, test the mass airflow sensor readings with the scan tool, check fuel pressure, as well as few other things. Read more about Mass Air Flow (MAF) sensor. If the new mass airflow sensor is installed, make sure the air filter is installed properly, as the sand particles coming with the unfiltered air can damage the airflow sensor.

P0171 - System Too Lean (Bank 1)

The code P0171 means that the air-fuel mixture coming in to the engine is too lean, or there is too much air and too little fuel. This could be caused by a number of reasons, such as low fuel pressure, plugged fuel filter, worn fuel pump, vacuum leaks, exhaust leaks, etc. For example, a Toyota Technical Service Bulletin (TSB) for 2003-2008 Toyota Corolla and 2003-2008 Toyota Matrix lists a leaking intake manifold gasket as a possible reason for the code P0171 along with some other codes.
From my experience, problems with a mass air flow (MAF) sensor are fairly common; sometimes the mass air flow sensor just gets dirty and gives false air flow readings causing lean condition and the code P0171. Read more about Mass Air Flow (MAF) sensor.
The code P0174 - System Too Lean (Bank 2) means the same, only for the Bank 2. Often the codes P0171 and P0174 are set together. For example, a Ford TSB for a number of late 90-s Ford vehicles describes a problem with MAF sensor contamination where both codes, P0171 and P0174 would appear together.
The code P0171 is very common, so we dedicated a separate page for this code. Read the full article: P0171 - System Too Lean

P0173 - Fuel Trim Malfunction (Bank 2)

See the code P0170.

P0174 - System Too Lean (Bank 2)

See P0171.

by Vlad Samarin