1994 model 940 with Bendix/Regina...Unknown mileage on 02 sensor. I removed it and replaced it with a "Bosch universal 3 wire" 02 sensor, splicing the wires to the original harness. The old sensor had a white fluffy tip with soot around the base of the sensor; that is supposed to mean "silicone poisoning". I have never used these products on the engine, but previous owner may have. New 02 sensor set "check engine" oxygen sensor code. I deleted the code but it came back. Meanwhile the car runs smooth as silk. Contacted IPD and they told me this sensor is specific to the Bendix/Regina and to 1994 year! Gulp. Ordered the Volvo part from Brentwood Volvo in St. Louis. After probing the locking plug on the firewall to check that the voltage indeed was swinging from .2 to .8 volts, the "check engine" light turned itself off. This is a locking type automotive plug that seems pretty foolproof - all my splicing was done down near the 02 sensor...Mysteries.
-WF