Q: What are the common things to check when the check engine light comes on?

When the Check Engine Light illuminates, it indicates that Diagnostic Trouble Codes(s) have been stored in your car’s engine computer. The codes themselves have to be downloaded in order to know where to begin the diagnostic. The most important thing for consumers to understand in interacting with their mechanic is that the codes do not necessarily identify the failed part, or indeed any failed part.

Codes are best thought of as the consequence of the engine computer not getting the input (data or voltage) that the computer "expected" or perhaps no input at all. This is important because although it is often "sensors" that are supplying the data to the engine computer, just because the signal has a problem, it doesn’t mean that the sensor (referred to in any given "code") is bad. Instead, it could simply mean a wire is broken, or there is a bad connection to the sensor or it could even mean that some other malfunction, that is NOT even being reported as a "code" is causing the sensor to start reporting "out of range".

So, to answer your question, first the codes have to be downloaded but those codes just provide "clues", albeit important and critical, from which the mechanic starts his or her actual diagnosis. In practical terms, this means that any "device" identified in a code has to be separately tested before its status (whether it works or not) can be determined.

Regarding your question of common things to check, there are many studies of the frequency of malfunction codes and they all have different results because there is some variance from car model to car model. However, one major study showed that the top five reasons for the Check Engine Light to illuminate were traceable to these codes: P0420 - Catalyst System Low Efficiency, P0171 - Fuel Trim System Lean Bank 1, P0401 - Exhaust Gas Recirculation (EGR) Flow Insufficient, P0174 - Fuel Trim System Lean Bank 2, and P0442 - Evaporative Emission (EVAP) System Small Leak Detected.

These five codes are not uncommon faults but again, your experience at the moment could be completely different. If your Check Engine Light has illuminated, it is recommended that you schedule a diagnostic with a certified technician, such as one from YourMechanic.