Q: Mercury Gr Marc 2001 "CHECK ENGINE" light turned on. What do I need to get checked

Hello. You’ll need to attach a Diagnostic Code Reader (aka, a "scanner") to a terminal beneath the dashboard of your car, which will read out Diagnostic Trouble Codes (DTC’s). These Codes usually offer critical clues as to what the underlying issue is likely to be. DTC’s are typically clues, and clues only as further diagnostics and testing of individual components is almost always required. DTC’s do not necessarily indicate exactly what part has to be replaced or what service has to be performed. You would need to have a professional technician inspect the Check Engine Light for you in order to have this corrected.