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I've seen NOX failures in several old 700s lately and with those, the cause is a bad cat. conv. diagnosed by removing the test plug before the cat, testing before the cat and then comparing those specs to those at the tailpipe. If it's the same before and after, the cat. conv. is NG. I havent yet seen 850s fail but it's possibly what's happening with your car, even though I'd think that it might set a fault code because 850s have 2 O2s to measure cat. conv. efficiency. Volvos VST (scan tool) can also monitor the O2s and could then determine the cats efficiency as well.
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