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Old Sun, Mar-01-2009, 02:38:42 AM   #2
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Join Date: Oct 2003
Posts: 564
Reputation: 0 kmurph will become famous soon enough
Location: Pittsburgh, PA

United States


With regards to the rear O2s, it's my understanding that they need to replaced in the correct order, however, as you probably know, it would not affect performance either way since they only monitor emissions - you might just throw a SES light.

Also, I think they both have different cord lengths, so they might only reach to correct plug at the valve cover...I would think that the best thing to do is to tag them or remove them one at a time to verify which one goes where...

I've tried to replace the pre-cat O2 sensors on my car from underneath, but have had no sensor is easy to get to while the other is not. I tried a 22mm o2 sensor crows foot socket with an extension, but it was too tall and didn't work in the tight space. I plan on trying a lower profile crows foot socket.

From where did you replace the pre-cat O2s? From above or below? I haven't removed the heat shield from the top, but don't want to if I'm wasting my time. Any help would be apprecited...btw, I have stock headers.

2007 Audi RS4 / GMG Exhaust
2001 BMW M3 - sold
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