Why Check Engine light comes on?

Updated: September 26, 2013
Check Engine - Service Engine Soon light
Check Engine or
Malfunction indicator light (MIL)

The Check Engine or Service Engine Soon light or an icon resembling the engine is called, in technical terms, the Malfunction Indicator Light or MIL. The MIL light comes on when your vehicle's computer detects a fault that can affect your vehicle's fuel economy and emissions. There is no button to reset the Check Engine light, but if the fault that caused it doesn't happen again during your next few trips, the light will eventually turn off by itself. If the MIL light stays on, have the problem checked out, because in some cases, the problem could get worse if not repaired in time. Think of the Check Engine light as an early warning sign. To check why your MIL light came on, your dealer or mechanic will have to diagnose your car with a special scan tool.

• What to do if my "Check Engine" light comes on?
• How the "Check Engine" light diagnosed?
• How the engine computer works
• Is it safe to drive with the "Check Engine" light on?
• Will disconnecting the battery reset the "Check Engine" light?
• How long does it take for check engine light to reset?
• Can overfilling the gas tank cause Check Engine light to come on?
• OBDII Diagnostic Trouble Codes (DTC)
• Freeze Frame
• I have a scan tool. How to scan a car for codes?
• Where to buy a scan tool or a computer software
• Tips to prevent the Check Engine light from coming on

Q: What to do if my "Check Engine" light comes on?

A: If the "Check Engine" light came on soon after a fill-up at a gas station, check if your gas cap is closed properly. Secure the gas cap if it wasn't tight, and if there are no other problems, the "Check Engine" light should reset by itself after a few trips. If the gas cap was tight, there is some other problem with the engine, transmission or emission control system components in your vehicle. You should have your car computer scanned to see what is wrong. It could be some minor or intermittent glitch; but it also could be something that can cause more damage to your vehicle if not repaired in time. Typically dealers and auto repair shops charge a diagnostic fee for scanning the car computer. Some auto parts stores and auto repair shops offer to scan your car for free, in hopes that you will buy parts or do the repairs at their shop. Google 'free check engine light scan' + ' your town' to find a shop that will scan your car for free. Once you know the code, you can google it again along with your car make and model, e.g., P0171 Ford Explorer. Do a little research and you will have an idea what is the problem and what to do - somebody probably already experienced the same problem with the same car and posted the information. We also have common check engine codes listed along with examples and common repairs; check here: OBDII trouble codes. If your car is not too old, have it scanned at your dealer, because it's possible that the problem is still covered by the emission warranty and can be repaired for free or a minimum charge. The US Federal Emission Warranty covers major emission control components for the first 8 years or 80,000 miles. Check your warranty booklet for details, or call your dealer. Again, doing some research ahead of time can help in case the dealer refuses to warranty your problem.

Q: How the "Check Engine" light is diagnosed?

Scanning the car computer for check engine codes
A technician scanning the car computer
with a scan tool

A: Once you take your car to a dealer or repair shop, a technician will hook up the scanner to the car's diagnostic connector and retrieve the stored diagnostic trouble code or codes, as well as the freeze frame. Then, he (she) will look up the code in the service manual provided by a car manufacturer. The service manual contains the list of diagnostic trouble codes (around a few hundreds) and describes what each code means, how to troubleshoot it and what parts need to be tested. The diagnostic trouble code itself doesn't tell exactly what component is defective; it only indicates where to look, what engine parameter was out of normal range. The technician will have to perform further testing to pinpoint a defective part. Once the repair is completed, a technician will reset the "Check Engine" light. If you have necessary skills, a good scan tool and the repair manual, you can diagnose the problem yourself. Read more about this on the next page.

Q: How the engine computer works?

Engine computer or the ECM
The Engine Computer or ECM

A: All modern cars have a computer (in the photo) that controls the operation of the engine, automatic transmission and emission control systems. This computer is usually called the Electronic Control Module or ECM (SAE term: Powertrain Control Module or PCM). The main purpose is to keep the engine performing at the most efficient level while keeping the exhaust emissions low. To achieve this, the ECM constantly adjusts the engine and transmission performance according to the speed, load, engine temperature, gasoline quality, ambient air temperature and other conditions. We usually don't notice how it works, but from the moment you turn the ignition on, everything is controlled by the computer. All passenger cars and trucks sold in North America since 1996 are OBD-II compliant. The OBD-II or On-Board Diagnostics version II standard among other things requires the vehicle computer system to have a self-testing capability. The ECM constantly monitors all the sensors and periodically tests electronic components and emission control systems. If the ECM detects a problem with some of the electronic components or the signal from one of the sensors is out of normal range, it turns the Malfunction Indicator Light or MIL on. At the same time, it stores the correspondent diagnostic trouble code (DTC) in its memory. For some problems, the ECM also stores the freeze frame of the engine parameters at the moment the fault was detected. Read more about the freeze frame on the next page.

Q: Is it safe to drive with the "Check Engine" light on?

A: It really depends what the problem is, what caused the "Check Engine" to come on. It could be something really minor, like a loose gas cap, but it also could be a more serious issue that needs to be taken care of as soon as possible. In worst cases driving with the "Check engine" light may cause more damage to the vehicle. Here is a very common example:
Often, due to a bad mass air flow sensor, vacuum leak or some other reason, the air-fuel mixture entering the engine becomes lean or there is too much air and too little fuel. The engine computer tries to compensate by adding more fuel, but it only can compensate within a certain limit. If the engine computer can no longer compensate the lean condition, it turns on the check engine light and store the corresponding code in its memory, typically P0171 - System Too Lean (Bank 1) or P0174 - System Too Lean (Bank 2). If caught in time, this problem may require a simple repair and more troubles could be avoided as the lean air-fuel mixture causes pre-ignition or detonation, and this could lead to serious engine problems. If your check engine light came on, I certainly recommend to have your car checked out as soon as possible to be on a safe side. If the Check Engine light is flashing, it means that the engine computer (ECM) has detected that your engine is misfiring. Driving with a misfiring engine could damage your catalytic converter, which is a very expensive part. Often the misfiring could be caused by bad spark plugs and wires; if you haven't done a tune-up in a while, it might be a good idea to do it now for a start.

Q: Will disconnecting the battery reset the "Check Engine" light?

A: Disconnecting the battery may reset the Check Engine light in some cars, but it will come back anyway if the problem is still there. Besides, the problem may still be covered by the emission warranty (typically 8 years or 80,000 miles) and repaired free of charge by your dealer. Also, the readiness code will be erased, which may prevent your car from completing an emissions test (I/M). The readiness code is an indication that certain emission control components of your car have been self-tested.

Q: How long does it take for check engine light to reset?

A: If the problem that caused the check engine light to come on is fixed or no longer exists, the check engine light will turn off. For some faults, it may take just a few minutes of driving; for other problems, it may take a few trips. This is because it takes time for a car computer to re-test all the components. If the Check Engine light doesn't clear itself after a couple days of driving, the problem is most likely still there.

Q: Can overfilling the gas tank cause Check Engine light to come on?

A: Yes, overfilling the gas tank can trigger the "Check Engine" light to come on. Modern cars are equipped with the Evaporative System that prevents gasoline vapors from escaping into the atmosphere. When we overfill the gas tank, the excess gasoline can enter the part of the Evaporative system called the Charcoal Canister, which is designed to absorb gasoline vapors rather than raw fuel. This can cause some problems with the evaporative system that can trigger the Check Engine light.
Don't overfill the tank past the first click of the pump. Check your owner's manual.