The OBD-II system monitors various sensors and components. A malfunction in any of these could trigger the light. Potential issues range from relatively minor (loose gas cap) to major (catastrophic engine failure). Some examples include:
* Sensors: Oxygen sensor, mass airflow sensor, crankshaft position sensor, camshaft position sensor, throttle position sensor, etc. Each of these can fail in different ways.
* Emissions System: Problems with the catalytic converter, EGR valve, EVAP system (fuel vapor emission control).
* Ignition System: Spark plugs, ignition coils, wires.
* Engine Components: Problems with the engine itself, like vacuum leaks, faulty fuel injectors, or internal engine damage.
* Transmission Issues: While less likely to directly trigger the check engine light, transmission problems can sometimes indirectly affect sensors and cause the light to come on.
To determine the precise cause, you need to use an OBD-II scanner to retrieve the diagnostic trouble codes (DTCs). These codes provide clues, but even then, further diagnosis might be needed to pinpoint the exact problem. Even with the same DTC, the underlying cause might vary slightly between cars.
Therefore, it's impossible to give a precise number, but the possibilities are numerous and span a wide range of severity.
