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 the exhaust emissions. There is no button to reset the Check Engine light, but if the fault that caused it doesn't happen again, the light will 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.

• How the OBDII system works
• What to do if my "Check Engine" light comes on?
• Is it safe to drive with the "Check Engine" light on?
• Can overfilling the gas tank cause Check Engine light to come on?
• Tips to prevent the Check Engine light from coming on

How the OBDII system works

All modern cars and trucks have a computer that controls the operation of the engine, automatic transmission and emission control systems. This computer is usually called the Electronic Control Module or ECM. 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 parameters 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. On some cars, even after the car is shut off, the ECM runs some tests to check some of the components.
All passenger cars since around 2001 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.

Once you take your car to a workshop, a technician will hook up the scanner to the car's OBDII diagnostic connector and retrieve the stored diagnostic trouble code or codes. 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

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 issue; but it also could be something that can cause more damage to your vehicle if not repaired in time.

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, but it also could be something more serious 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. 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.

The trouble codes on all OBDII cars are standardized and each code has the same meaning on all OBDII cars. There could be some minor differences in the way different car manufacturers interpret the same trouble code, but the basic meaning is the same. A typical OBDII trouble code starts with a letter that is followed by four digits. The letter "P" stands for powertrain, the letter "B" for body. For example, if the engine cylinder number 2 would misfire, the car computer (ECM) would turn on the "Check Engine" light and store the diagnostic trouble code P0302 in its memory. If you'd connect the scan tool, it would read something like: P0302 - Cylinder 2 Misfire Detected. Overall, there are a few hundreds trouble codes for the powertrain, but only about 40-50 codes are very common. 

Tips to prevent the Check Engine light from coming on

Don't overfill the fuel tank. Fill up till the first click of the pump. Overfilling can cause problems with your vehicle's Evaporative emission control systems (EVAP) resulting in the Check Engine light coming on.

After filling up, make sure the gas cap is tight and the gas cap strap does not get caught under the cap.

Change engine oil regularly. Long intervals between the oil changes do not directly cause the Check Engine light (MIL) to come on, but many MIL-related problems stem from lack of oil changes.

After checking or replacing the air filter, make sure it's installed properly. If unfiltered air is allowed into the engine it can cause damage to the vehicle's mass airflow (MAF) sensor, which will turn the MIL on. It's best to use only OEM or known good quality air filters.

Lack of tune-ups is another common reason for the Check Engine light to come on. Old spark plugs, dirty throttle body, bad ignition wires all can cause engine problems with the Check Engine light coming on.

The corrosion in the wiring and connectors of the car computer system is another major cause of the Check Engine light problems. Your car or truck is full of electronics with wires and connectors located in many places throughout the vehicle, including areas behind the kick panels, under the carpet, in the engine compartment, underneath along the frame, near the gas tank, etc. Anything you can do to keep the electronic modules and wiring free from corrosion will help avoid related problems. For example, if your engine undercover is damaged, it can allow moisture into the engine compartment causing corrosion. Similarly, moisture inside the car can cause corrosion of the wiring under the carpet. Keep the electronics dry.

When doing repairs, use original parts when possible. Sometimes the Check Engine light comes on because some aftermarket parts like oxygen sensors, mass airflow sensors or catalytic converters can have compatibility issues.