P0420 keeps coming back

I replaced the cat and one of the O2 sensors, but the CEL comes back on after a while. The only thing that has slowed its reappearance is Berryman's Chemtool. Any ideas?

If you're Legacy was newer, I'd vote to modify the AF#3 Learn/Correct thresholds to effectively disable rear O2 fueling contributions and disable the P0420 DTC.

 

However, in your case, I'm assuming you replaced the front AF sensor and the cat, not the Rear O2 sensor, correct? Or did you do the cat and Rear O2, but not the upstream AF sensor?

 

For the P0420 NOT to rear its ugly head, you need to have a good correlation between what the AF sensor and Rear O2 sensor are reporting during their test cycles.... and to help them achieve that, the cat must be good and in a good location too.

 

What cat did you replace the stock one with? Enough cells and length of the substrate?

Did the car have a single or a dual (back-to-back) cat setup under the stock heat-shields?

Small exhaust leak. Usually at the y pipe split. On my 05 I couldn't tell there was a leak until I sprayed soapy water. Exhaust has to be relatively cool.