engine electronics message?????

[cerbomark]

On my 98 e 430 the engine light came on and then the message stating" check engine eletronics". The car still runs fine. I called my dealer and he told me to make sure the gas cap was on tight. I told him it was but also told him two days prior I had filled up with gas and had left the car running (which I never otherwise do).He stated that was the problem and just to drive the car a few days and the light would re set. Whats this all about??? Thanks.