Thread: 2nd oxygen sensor before or after cat?

I just had a cat put into my straight pipe and the guy welded the cat in after the 2nd oxygen sensor which I didn't see until I got home and looked underneath.

Isn't the cat supposed to go before that sensor? Does it matter how far downstream that second sensor is? I might just have them leave the cat where it is and move the sensor and Ill just extend the wires.

Will I get back some gas mileage by putting it back after the cat? I have I/H/E.

As I understand it the second oxygen sensor reads on how efficient the catalytic converter is working. So, it will make no sense to have it before the converter.

Right after the cat, actually your cat should have a bung on it, unless it's a universal.

The second O2 is used to check the CAT, and if your front fails the FSM says that secondary can be used.

Thanks for the reply guys, you confirmed my suspicions. It's one of those universal cats so Ill have to go back tomorrow to have the guy fix his mistake.