Some weeks back, I replaced the front oxygen sensor 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, 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.