Engine Basics: the CEL (Check Engine Light)

Depending on what type of car you drive, the check engine light can either be a continual (comforting?) presence on your gauge cluster, or an ominous herald of doom whose orange light illuminates your deepest car-related fears. It’s not either, and there are a few things that you should know about this often misunderstood warning light. First of all, it comes in a couple of varieties. Some cars have a light that literally spells out “Check Engine,” others have an icon that looks like an engine. Usually they are yellow or orange. If you’re unsure of what to look for, your owner’s manual will have an illustration of the icon.



So what does it mean? The CEL means that some system on the engine is working improperly. The computer on your car monitors the engine through a number of sensors, with the most having to do with the cars exhaust (emissions control) system. There are sensors that measure the composition of the exhaust, for example, to make sure the engine is complying with emissions requirements for state and federal laws. Some of the sensors look at aspects of how the engine is actually running. In any event, when one of these sensors detects a condition that is outside its acceptable range, it will send a coded signal to the computer, which will in turn display a CEL.

In some cars, the CEL will flash on and off if a really serious error is detected. If you see a flashing CEL, I would recommend safely stopping the car and consulting your owner’s manual. When in doubt, or if the car feels unsafe to drive, please use common sense and don’t drive it. Call a tow truck or your mechanic.

Usually, the CEL stays on in a steady glow. It’s really just there to say that “something is wrong! It could be one of many things! Get it looked at!” The CEL tells you almost nothing about what is actually going on, unless you can read the code that the sensor sent to the computer. This is exactly what happens if you have a car built after 1996, and you take it to the mechanic. Starting with the 1996 model year, all cars were required to have an OBD-II (On Board Diagnostic [System] – II) plug, which is a universal code that is standardized among all brands. So whether it’s a Mazda or a Jaguar, an OBD-II reader device will be able to download the error code from the computer. (Older cars may have non-universal readers for them, but in any event, your mechanic should be able to read any CEL code.)

For example, let’s say you have a Mazda Miata from 1996. If you had an OBD-II reader, and you plugged it in, imagine it displayed the code “P0126.” You’d have to refer then to the list of codes, and you’d find out the code meant “thermostat stuck open.” That is not necessarily a serious fault, but it could affect the way your car ran in cold weather, so it would be a really good idea to take it in to a shop and have them look at the car.

Of course, without a code reader, all you see is the light. And the point I want to make is, the light doesn’t tell you very much. With our hypothetical Mazda code above, you could in all likelihood drive your car to a mechanic to have them look at it. But the light isn’t there to tell you whether to drive it or not. I wish I had better advice, but again, I would say to exercise your best judgment, and if you see the light, bring it to a mechanic in a safe and responsible manner. (That might mean towing it!)

The one thing I CAN tell you without a doubt is that the wrong way to respond to a CEL is to tape a piece of paper over it! I know folks who have done that. The CEL means business, so respect it!