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· Premium Member
8,086 Posts
Early, but not at all uncommon to fail. If the connections all look good, I would just go ahead and replace fact replace both pre cat O2 sensors, and save yourself some work. My 2001 540 had one pre cat O2 sensor failur at around 30k miles and three years. I've had them fail on other cars (usually the heater circuit on the O2 sensor) well before 50k miles. These are all Bosch brand oem O2 sensors too, just like used on BMWs.
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