Engine light may or may not indicate a disaster
QI remember you said there can be dozens, if not hundreds, of reasons my “check engine” light might come on. And since there's no way for me to know which one it is without it being checked by a repair shop, how do I know it's a serious problem or not? I think I heard somewhere the gas cap being loose is a common cause; what others are there? — Vinnie Salcida
AAside from a flat tire or dead battery, an illuminated “check engine” light — also known as a malfunction indicator lamp, or MIL — is probably the most dreaded and common automotive quirk to wreck someone's day. This amber lamp indicates the emission control system, engine or transmission has incurred a fault that will cause exhaust emissions to rise above allowable values. In many cases, the vehicle will drive flawlessly. In other cases, misfiring, stalling or other drivability symptoms can lead to bigger problems.
In most cases, a continuously glowing MIL with normal engine and transmission performance can be prioritized as a sometime-this-week visit to a technician. A flashing MIL is a different story: The vehicle should be driven as tenderly and briefly as possible, as a severe engine misfire is occurring, which can damage the catalytic converter, among other issues. If the lamp stops flashing and misfire symptoms, such as shuddering or a thumping engine, abate, one may tread further.
According to the California Bureau of Automotive Repair, the most common “check engine” causes and diagnostic trouble codes are:
“Catalyst system efficiency below threshold” (P0420): This means the catalytic converter is no longer functioning efficiently and probably requires replacement. This will not adversely affect vehicle operation, but exhaust emissions aren't being properly treated.
“System too lean” (P0171 or P0174): Lean means the engine is not receiving enough fuel or is admitting too much air, and may be indicated by stumbling, pinging or a loss of power. Engine damage is possible with this situation if the vehicle is driven under demanding conditions.
“Exhaust gas recirculation flow insufficient detected” (P0401): This can cause combustion temperatures to rise excessively, leading to rattling, knocking sounds or pinging. Reducing speed and avoiding hills and hot-weather driving can help mitigate possible engine damage prior to repair.
“Random/multiple cylinder misfire detected” (P0300): If noticeable engine shuddering or reduced power is evident, get this fixed as soon as possible. Engine misfire causes a host of problems, as noted above.
“Evaporative emission control system leak detected” (P0442, P0455, or P0440): This means fuel tank or vapor storage canister vapors are escaping to the atmosphere. Check for a loose gas cap, but remember that it may take several days for the lamp to go out. Vehicle performance is unaffected.
“O2 sensor heater circuit” (P0135 or others): A faulty heater in one of the oxygen sensors is prolonging sensor warm-up or allowing cool-down. Vehicle performance is unaffected.
A neighbor with an inexpensive OBD-II scan tool or code reader can retrieve the stored diagnostic trouble code, and a quick Internet search of the code will yield sufficient information to allow repair prioritization.
Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at firstname.lastname@example.org.