suby dealer said one o2 sensor was bad, but both should be replaced, in pairs, but they said the # 2 was bad, and they do not need to be replaced in pairs, they stated that, unlike o2, which should always be replaced in pairs, a knock sensor, can be done indvidually... because, they work with a "one trace" resistence factor, where as the o2, are different...i didnt know what to say when they said that...except...um, ok thanks!! lol