Most often, the check engine light on your dash is interlinked with diagnostics error codes. Simply put, if the ECU has determined that something's off with your car, it would store error codes that you can use for diagnosis later on. But, in so doing, the ECU typically lights up the check engine light as an alert that there are some codes that you might want to check out. Granted, there are also certain situations where error codes can be stored, but without the check engine light appearing. Sometimes, this is because a previous repair job hadn't properly cleared out the existing error codes, causing it to appear once again if you plug an OBDII scanner in.