As a general rule, you can judge the severity of an intermittent problem by what it takes to make the light go off. The least serious is when the light turns off while you're driving. At that time the problem has stopped acting up for the moment. If it's more serious, even if the problem stops acting up, the light will stay on until you turn the ignition switch off and restart the engine. The light will stay off until the problem occurs again.
When it's still more serious, the light will be on any time the engine is running, even when the problem hasn't occurred. With the really serious codes, the light will be flashing. That means stop the engine right away. Too much raw fuel is going into the exhaust system and is going to overheat and damage the catalytic converter.
About half of the fault codes do not cause the Check Engine light to turn on at all. Only those problems that could adversely affect emissions must turn the light on.
Friday, January 24th, 2014 AT 9:13 PM