2001 4.6L. Will replacing the stock pre-cats with straight pipe and LEAVING the stock main cats in place cause a check engine light? I ask because my main cats have the downstream O2 sensor installed in the middle of the cat itself, as opposed to after the cat. That leaves half of the stock cat to do the work that needs to be done before reaching the O2 sensor. Thoughts? Thanks.

 

you can always pull em off and see what happens,and if you do get a mil you can put a 1/4 ohm resistor in the signal wire(should be the gray wire) on the downstream o2 sensor

 

Thanks man, I prefer not to head that route, but I know it's an option.

 

On my 2000 flareside 5.4 I took the stock cats and pre cats off and replaced them hi-flow cats (one on each side) My CEL has never came on.