Try unplugging the heater on the O2 sensor (the two wire plug). If it is defective, it could short to the electrode in the O2 sensor and cause your fault.

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!