Magnum Swap, which 02 sensor to use, pre-cat or post-cat

dano said

Depends. If the truck/van had one cat then one o2 was pre-cat and the other was post. If they are both missing then you will get a code. If dual exhaust then they are both pre-cat. The post-cat o2 needs to be defeated somehow as it is looking for converter efficiency.

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I did a magnum swap in my Barracuda in 2012, ran factory pcm, no cats. The way I tricked the pcm was to use a non foul sparkplug adapter in down stream O2 hole, then screw down stream O2 cell in. What happens is the down stream O2 cell will report a change but since it is not in the exhaust stream it will not read correct. The pcm sees a signal and never knows the cat is gone. I also did this when my cat on my 2000 dodge dakota went bad, removed cat straight pipe, used this trick and presto no codes.