That is a very common misconception that codes can only be read when the Check Engine light is on. A lot of inexperienced mechanics even believe that. In fact, there are well over 2,000 diagnostic fault codes that can be set just in the Engine Computer, and only about half of them refer to something that could adversely affect emissions. Those are the codes that turn the Check Engine light on. There are over 1,000 engine codes that never turn the light on.
You can get an idea of the severity of an intermittent problem by how the Check Engine light acts. If the problem stops acting up while you are driving and the defect is relatively minor, the light will go off when the problem clears up. If it is more severe, the light will "latch" on, and if the problem clears up, the light will not turn off until you stop and restart the engine. For even more serious problems, the light will always be on when the engine is running, even if it is not acting up at all. The most serious problems are when the light is flashing. That means too much raw, unburned fuel is going into the exhaust system where it will burn in the catalytic converter and overheat and damage it. You are supposed to stop the engine right away.
Friday, October 7th, 2016 AT 8:16 PM