Internuts: 96MB E300 Check Engine Light (CEL) on

My friend, who used to lease a E4xx, came to help me figure out which buttons for what. He tested some buttons. He also turned on engine, simutaneously pushing the desiel pedal, and then I heard some squeak sound. I knew you don't need to push the desiel while turning on the engine. After this incident, whenever I turn on and drive, the "Check engine and electronic" light is always on. I don't feel anything wrong while driving, but also not so comfortable. Any input? Thanks in advance!