I took my 96 850 to the volvo dealer to investigate an engine light and they told me that the "aft O2 sensor" was slow to respond. I did not have them change it because of the high cost. Shortly later my car was due for emissions test and the fault code came back P0133 O2 sensor circuit slow response bank 1 sensor 1. How do I verify which sensor this is, or do I just replace both of them to make sure? Thanks in advance for any help. Andy.