A description of why a vehicle check engine light comes on and the OBDII monitoring system. Includes an explanation of myths and facts surrounding check engine lights and servicing them
There is a lot of confusion and misinformation out there about why a check engine light comes on. 1996 and newer vehicles have an OBDII computerized monitoring system. OBDII uses different sensors to gather information, vehicle speed, air temperature and engine temperature, for example. It feeds that information into the vehicles computer, commonly called the ECM. The ECM then adjusts things like air/fuel ratio so that the vehicle runs well. If OBDII detects a fault in one of the sensors, it will turn on the check engine light to let you know that something is wrong. Here are some myths cleared up about why the check engine light may be on and what you should do about it.
Myth: My check engine light is on, my oil must be low
Fact: A low oil level condition will not set your check engine light. That light will come on if something is wrong causing your vehicle to produce more one and a half times more the emissions than it was designed to produce. It will not come on if your oil level is low, if you need windshield washer fluid or if there is a problem with your brakes. It also will not come on if your oil is due to be changed, you may have a maintenance reminder light come on, but it will say something like "Maintenance Due".
Myth: The code in the computer was for an oxygen sensor so I should replace it.
Fact: Your check engine light came on so you stopped at Auto Zone and they scanned the computer for you for free and said that the code was for a bad oxygen sensor, so they sold you a new sensor to replace. First of all, there is likely more than one sensor on your vehicle, are you sure you got the right one and are you sure you are replacing the right one? An oxygen sensor code, or any code for that matter, can come on for a number of different reasons. The oxygen sensor code, for example, could be caused by a problem in the wiring for the sensors or a plugged catalytic converter or a number of other things. A check engine light will direct you to what system is throwing the code, but it will not tell you what caused it. Further diagnosis is necessary, just replacing parts is expensive and the light may come back.
Myth: My check engine light came on, I should have the vehicle towed
Fact: So many things can make that light come on from a bad MAF sensor to a gas cap that isn't seated properly. The rule of thumb is if the check engine light is on and flashing, pull over and have the vehicle towed. If it is on and steady and the car isn't running so badly that it isn't safe (bucking, hesitating, stalling) then you can drive it. If the light is flashing it is an indication of a misfire so severe that there is a danger of doing damage to the catalytic converters.
Myth: My check engine light has always been on, there's nothing wrong with the car, the light just stays on.
Fact: The light will not come on if there isn't a problem. It may be a problem that is hard to find, it may be a evaporative fuel leak that is so small it's hard to detect, but if you bring the vehicle to the right technician with the right equipment it can be fixed. Depending on the code, not having it fixed can lead to poor gas mileage, carbon buildup in your engine and further damage done to other vehicle components eventually rendering the vehicle undrivable.
Myth: You can disconnect the battery or pull the fuse for the ECM and the check engine light will go out.
Fact: The light may go out briefly, but it will come back, usually within 50 miles or less. Clearing the light will not fix the problem, as you drive it the vehicle the OBDII system will run tests for each sensor, once the same problem occurs again the light will come back on. That's why it's always best if you have a check engine light issue fixed to have the technician NOT turn out the light, but to run a drive cycle if possible so that the light will go out on its own. That's how you can be sure that the actual problem was fixed.