check engine light and message center

97 E 320, 24K miles, original owner. Message center comes up with check engine electronics and check engine lite is on. Bring to dealer, he can't find any code that would describe where the problem is, or what to correct. Tech, cancels the check engine message and we drive away. Next day, bingo its on again. Went back, the tech did some service on that "hot wire" thingie in the intake track and canceles the messages again. Next day, its back. What gives with these messages. Car is running fine with lots of oil and water. Tech cannot find any codes to lead him to correcting the problem. Appreciate your advise. thanks, Mick