Ok I have not had a reading of 2&3 That has any movement when I put my digital meter on percent, IE the reading is a constant 50.2% to 50.3% from cold to hot or warmed up to operating temp. It had a check eng. lite so I put a new ox sender in and have no fluxuation in the test reading as normal. can some one direct me toward likely reasons? thanks.

Is the car a California version? Blink pushbutton & red LED on the diagnostic connector?

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no it is not ca. car. the Duty cycle reading that I have had in the past has always had a fluxuasion when I check it and set it around 49% bounces from that to 51% some times higher. but after this ox sensor, it is riveted to a fixed number. thanks memory

Maybe the new sensor is not hooked up right? Unplug it an see if there is a change. Mine fluctuates greatly, but I read it just as voltage, then add then up and divide by two.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

I had this same problem with my 300E. The engine ECU failed in such a way that the car was always in "open loop", thus operating at a fixed duty cycle value all the time and never entering "closed loop".

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1991 300E - 212K and rising fast...

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?