"So it seems there's a problem with an oxygen sensor. I know there are two--can anybody tell me how to test them? How hard are they to replace?"

You can use your scan tool. They should fluctuate around 0.5 volts. The front sensor should have more fluctuation than the rear and the movement should be fairly rapid. They are easy to replace but expensive and the wiring pigtail connector is in the engine compartment on the drivers side. Not the easiest of places to get to.

"My son made the first entry, and I think mixed up the numbering: the spark plug that was black was in the cylinder closest to the firewall, which is #6, right? Might the O2 sensor make the cylinder misfire, or is that another problem?"

A bad O2 sensor would not cause the condition in one cylinder. Could be a bad primary wire to the coil pack, a bad coil pak, spark plug, or fuel injector.

Reset the codes, then pull the first on ethat comes back and go from there.

DEWFPO
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1998 S90 068,300 and 1995 964 152,100