Reposting here, where it belongs, instead of my original erroneous post (too many Volvos...)

Some weeks back, I replaced the front oxygen sensor on my 1996 850 because the trouble code that showed up from my $150 code reader was consistently "#1 oxygen sensor slow response" - until I got the "#1 sensor no response" code, that is. But the check engine light kept going on even after replacement of the sensor, with the "#1 slow response" code reading.

Finally gave up and took it to the dealer. The service writer told me that it's common for the rear sensor to go about the same time and to confuse the front one, thus causing the bad reading, without itself showing up as bad for anyone but the official Volvo code reader. He was right. Some folks on this forum had recommended replacing both sensors - if I had, I'd have saved about $170, between the higher dealer part price and the dealer labor.

"Experience - something that would have been very nice to have right before you acquire it."