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Q--If a faulty or missing gas cap can signal a “check engine” warning light, what else can? I suspect a low fuel quantity will as well, but how about a low oil pressure or low oil? Would low battery voltage initiate a check engine light? Just what parameters does this check engine light cover?
A--The short answer is that a problem with any emission control device that might cause emissions to exceed the certified emission levels by 150 percent can trigger the check engine light. A loose gas cap or broken fuel line or fuel vapor line or similar failure can allow vapors to escape. A failed emission control sensor such as the oxygen sensor, coolant temperature sensor, throttle position sensor, and so on will illuminate the check engine light. Anything that causes a misfire, such as a bad spark plug, fuel injector, or ignition coil can trigger the light.