2004 Toyota Tacoma needs new catalytic converter after replacing muffler?

After developing an exhaust leak in the muffler of my 2004 Toyota Tacoma my mechanic replaced the exhaust system from after the catalytic converter to the tailpipe - apparently this is a single assembly. Excellent, it’s quiet again! Very next day the check engine light is on and the code (P0420) indicates a bad catalytic converter. I trust my mechanic and have been going to him for years, but this seems to be too much of a coincidence. He said a bottle of stuff you dump in the gas tank to clean the catalyst might help, but it didn’t and the code has come back on even after a few resets. Is there something that could have happened during the exhaust replacement or an incompatibility with the exhaust system that is causing this error condition or do I need to suck it up and spend a lot of $$$ on new catalysts? Why no trouble with this until the exhaust repair?

I’ll add that the mechanic says that rear O2 sensor appears to be working correctly.

What you want your mechanic to do is compare the frequency of zero crossings of the front O2 sensor to the frequency of the rear O2 sensor. The frequency of the rear O2 sensor should be a significant factor less than the front O2 sensor. IIRC a capable scanner can provide that data.

If the front and rear O2 sensors are tracking together that indicates that the catalytic converter is not capturing exhaust oxygen which is used in the catalytic process to oxidize the unburned hydrocarbons and carbon monoxide.

Problems I would look for is no zero crossing of the front sensor or a lazy front sensor that does not switch fast enough to track the actual exhaust gas oxygen content. In fact IIRC your emission system may have a true fuel/air ratio detecting front sensor. Also check the wiring to make sure both sensor wires are intact to the PCM. One coincidence that a exhaust system replacement might involve is the use of a TIG or MIG welder on the piping while attached to the truck. If the sensor wires were not disconnected the high frequency voltage might have taken out the input circuitry of the PCM. Hope that is not the case.

Keep us appraised of the progress on this DTC P0420 situation – especially if you find the solution.