My S420 Check Engine Light has gone on recently and simultaneously the engine idle has dropped to an all time low. Upon starting, the idle speed is less than 200 RPM. When it warms up, it is about 450 RPM. I can't keep the car running when I start it due to the low RPM. When it is running however, it is surprisingly smooth and the speed does not fluctuate.

I checked the spark plugs (Bosch Supers) which were fine, air cleaners, and I found no vacuum leaks. I checked the resistance across the white leads of the oxygen sensor, it was 2.7 Ohms. What could be wrong? Any assistance would be greatly appreciated.

There's good info in the archives. Start with this:

check engine light

Sixto
95 S420
91 300SE
87 300SDL
83 300SD