| I experienced a simular problem with my 98" 540I. I took it to the dealer three times. Every time after about 3 to 5 days the check engine light would come back on. They ran the codes several times, and could not resolve the problem. However, upon an indept engine inspection a service tech noted some stressed knocksensor/spark plug wiring. He also noted that someone had performed some cylinder head work on the vehicle in the past. He suggested that is what probably created the wiring stress and causing the problem. He repaired them and I have not had the problem since.
Fruthermore, my check engine light comes on whenever my wife attempts to SAVE MONEY and burns 87 octane gas in our 328I. I immediately go to sunoco and fill it up with 94 and the light goes away.
I hope this helps.
Thanks,
JAB. | 
