I bought a 97 E420 about 3 months ago. It had 98K on it then, and has 100K now. Soon after I bought it I started having CEL problems and had it scanned at a local shop that works on MBs. Scan said oxygen sensors, so I decided to have all 4 replaced, since it was near 100K. Also had the plugs replaced. Everything was Bosch OE stuff. Still had problems, and the shop scanned it again and got codes PO 140 and PO 133, which they said were oxygen sensor codes (I'm not sure which bank). The shop said my best bet was to go to the dealer. I stopped by the dealer's to talk with a service advisor, and I was told that the problem might be the air mass sensor or the catalytic convertors, but probably not the control unit. Estimate to diagnose the problem was $100 - $150.

The car runs great, and it's obvious it has been taken care of by the previous 2 owners.

Before I get taken to the cleaners, does anyone have any suggestions about what I should do before I throw myself on the mercy of the dealer . . . who is not known to have much mercy!

Thanks for your help.