What are some reasons my check-engine light could be on?

While it’s true that many check-engine lights are caused by a problem within the exhaust system, there are literally hundreds of other reasons for a non-specific check-engine light to suddenly illuminate. To simply start changing sensors in the hope of hitting on the right one is a great way to waste time and money and still have a car with a check-engine light (which, technically, makes it unroadworthy).

If you are convinced the exhaust is the culprit, a likely suspect is the oxygen sensor which more or less sniffs the engine’s exhaust gas to determine whether the fuel/air mixture is correct and that the engine is burning cleanly. The sensors are usually screwed into the exhaust system near the catalytic converter. They’re not usually too difficult to change (assuming you can get the car in the air) but a better bet would be to have the car electronically scanned to discover the real reason the check-engine light is glowing.