I 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?
Aside 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. It may seem odd a transmission fault can affect emissions, but a shifting or torque converter clutch problem can make a difference.
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” lamp causes and diagnostic trouble codes are as follows:
“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.