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When we had this code on our daughters 2001 Outback ( 2.5L ), it was 10 years old. No exhaust leaks. Tried rear Oxygen sensor, still P0420. So I purchased an extender. Had to drill out the center for the Oxygen sensor to fit, then installed it. Takes rear sensor out of the flow. No more P0420. It was still on the car when we sold it 2 years later.

6 cyl or 4cyl? scan for and make note of any other codes including 'pending' codes. Clear the codes and see how quickly the P0420 comes back and any other codes as well. many people jump to changing the cat conv (and of course they do go bad) but, that code can be caused by exhaust leaks, vacuum leaks or bad wiring. The code on the 4 cylinder cars is only a 'nanny' code intending to monitor cat efficiency but is NOT used to alter any air:fuel ratios or other operating parameters.

Neat looking Leone. Careful with that plan, it can get out of hand quickly (in a good way)!! Cheers Bennie