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On my 03 zetec I have a long tube header. Im not sure which is upstream or downstream. Should upstream be the blue or green harness? Can someone with a cat verify for me on theirs?

Im guessing its the green one as it plugs into a harness higher up in the engine bay on the rad fan. The NTK O2 sensor I bought for the upstream has a white connector instead...
Just want to be sure I swap the right one out.
 

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FF's Night Security
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The one that went before the cat is the upstream the one after. If you already had the header on the car when you bought it and don't know which one it is then it's going to the one that comes out of the wiring harness up higher on the radiator fans where the wires are, the after cat one will be lower so you are correct in your guess lol.

NTK is a good O2 sensor they make the OEM motorcraft ones. Did you make sure you ordered the correct sensor? I don't remember what color mine were it's been a while since I replaced my sensors though I need to do it again since I popped up a O2 slow response code months ago.
 
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