I suspect this to be a proublem with the ECU, though I was looking for more morral support. I thought Doc had this one but I am slow to respond to the MB forum with every thing else going on at the present time. I do thank you for the pennies, and I hope you are having good luck with your ride. Memory you know I just rememberd that after working on my wifes dd a 91 190E 2.6 I used her old ox sensor and put it in to the 420 could this be the trouble are they unit specific? the 420 had a cel and it stopped after replacing with other ox.could this be the reason for the open loop?