I would replace just the rear sensor if that is indeed the problem. Since you bought the car used the front one may have already been replaced. If I recall correctly, the fronts generally last about 130-140k and usually go before the rears. On my 95 850, the front was replaced before I bought the car at 160k and the rear was original. Drove the car to 250k and interestingly never needed to replace the rear sensor....
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Will I buy another Volvo??? We'll see....