Last week I replaced the Air pump, valve, and hoses on my 1998 C70 HT. I also replaced the upstream o2 sensor at the same time since there was a CEL for that as well. Everything was great and worked like it should for about a week/300 miles. Then driving it yesterday, the CEL for the Upstream o2 sensor low voltage came on. I checked and the plug is still connected and the wires dont look damaged. How likely is it that I got a defective o2 sensor? Or is there something else I should be looking for?