62Blazer
Senior Member
It's not unusual for the check engine light to come on and there not be any obvious performance issues with the vehicle. That can be because the issue doesn't really affect the performance (i.e. gas cap light, etc...) but still needs corrected. It could also be because the computer can perform a lot of compensation and "cover up" issues so they are not real noticeable. For example it may be detecting small misfires (knocking) in the engine and retarding ignition to prevent them....while that may prevent the average person from feeling any misfires you can be slightly decreasing performance and fuel economy. The next question is why does the check engine light only come on every few days, weeks, or even months and then goes away when you clear the codes (i.e. disconnecting the battery clears the computer memory). If the codes coming back is always for the same reason that is because it's not occurring constantly, or it takes a certain number of occurrences to trigger the light. Using misfires as an example, if the check engine light would set every time it detected a single misfire the light would always be on. However it counts the total number of misfires and that has to equal a certain number in a certain time period before it triggers. Of course it could an issue that just sporadically occurs. Even if the issue only occurs once (or occurs enough to trigger a code) and then goes away the engine light will stay on for a certain period of time and will not just automatically go off when the problem disappears. Different issues/codes have different criteria for when the light may go out on it's own (usually a certain number of key cycles the issue not occurring).
