A few months ago I started getting the 153 code that indicated a rear O2 fault code and I had my indie mechanic replace it with a universal 4-wire Bosch sensor. This is a much cheaper fix than the OE Volvo O2 sensors that come with wires and connectors and my indie mechanic has done this to a few Volvos before and never had a problem. Lo and behold, the same code would come back almost every other day after I reset it. For some reason there was too much resistance going into the circuit and the ECU kept thinking there was something wrong with the O2 even though it was brand-new. We re-soldered it and rechecked the wiring several times and eventually was left with no choice but to replace it with the OE Volvo O2 sensor. This final move solved the problem and the code has not come back since. I'm sure there are plenty of owners who have gone the universal route and got it to work - I'm just pointing out that it might not work, as in my case. Maybe there was something wrong with the wiring or connector in my case and hence, replacing just the sensor did not fix the problem - I know it's rare, but just be on the lookout.