Bad O2 sensor will do this as well, I believe -- it doesn't respond to oxygen properly. Make sure it's connected and that the heater works (the other two wires to the sensor are the heater) -- it's only sensitive to oxygen when it's about 600F.

Otherwise, this system really doesn't set codes, it just fails to work properly, or quits altogether. Should sit at 50% with a bad or disconnected O2 sensor no matter what you do with the mixture.

Do a search and take a look at the discussion in the DIY section on engine controls -- I think something like this has come up before, but don't remember the thread.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!