Do we need to have that second O2 sensor in the exhaust piping? The reason I ask is because I know some cars don't use the second as their primary reading. Mine broke installing my new exhaust and was wondering if I should just leave it busted. I heard somewhere that all it does is monitor the cat convertor. I just want to make sure I'm not gonna run rich if I don't replace it. Oh by the way, I just replaced the first one(one in downpipe), so am I all set or what? Thanks guys....

Nope you dont need the second o2. It is not used for anything except for the ecu to check that your cat is working properly. It will bring up a check engine light, but won't affect performance. All a/f ratio is read from the primary upstream o2 sensor. I haven't had a downstream for almost 2 years now.

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2004 WRX - WR Blue Pearl
1997 Eclipse GS - Royal Sapphire Pearl

Thats what I wanted to hear.....thanks man.