engifineer

The secondary O2 has zero impact on how the car runs. It's only purpose in life is to tell the ECU if the cat is doing its job. The resulting code if this reading is off serves no other purpose than to tell you that the sensor or CAT may be in need of replacement. You run a defouler when you remove or gut a CAT. This pulls the sensor out of the flow, which reduces the amount of fluctation it sees in relation to the primary O2, which tricks the ECU into thinking the catalytic convertor is doing its job.

Now, if he put an anti fouler on the primary O2, it likely would throw off its readings (or cause a lot of hysteresis) and cause the fuel trim to be off since the ECU would be getting bad readings.

jwzimm

See post #5 in this thread. He refers to installing the defouler on the "one in the header". That would be the primary O2 sensor. Hence my concern. You are correct that installing the defouler on the secondary sensor would ave zero effect on engine operation.

darcyw

thank you all. I kept the defouler installed in the cat as recommended above. I appreciate the information. I'm usually pretty good at this stuff, but this one had me confused.

till the next one.

​​​​​​​darcy

AZS2KDancer

So in all this confusion, can a defouler be good insurance?

flanders

Yes, but at the same time you wont notice if your cat is going bad.
Except maybe loss of performance, but that's usually only when the substrate come loose and sits on it's side.