How Much Does My Check Engine Light Tell Me?

Your check engine light is a very useful diagnostic tool, and a good way to keep an eye on the heath of your engine, but not your car.

Your check engine light mostly reads from a number of sensors. These include oxygen sensors which let it know if the car is running rich or lean, temperature sensors which let it know if you are overheating, throttle, camshaft and crankshaft position sensors, and on almost all cars these days, ABS sensors that tell it wheel speed.

So, of something is going on which one of these sensors can read, your check engine light will come on and you or a mechanic can use it to help diagnose the problem. For example, a broken house line in the engine could be misread as a misfire which you need to replace your spark plugs or cool packs, instead of a $10 hose. The light only goes so far, knowledge is still key.

The light also tells you nothing at all about the mechanical systems on your car. Axles, brakes, suspension, most of your steering system: none of these things have sensors which tell you when they need repair or what they need, other than a few cars with brake pad sensors which don't work very well.

Your check engine light is a useful diagnostic tool, but it only covers a small percentage of repairs other than on the engine itself.