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96 toyota camry 4 cyl check engine code P0420

Replaced front exhaust pipe/catalytic coverter 3/06 with 100,000 miles due to check engine light code; now 120,000 miles 2 years later, light back on, code P0420 & toyota dealer says need new catalytic converter again. Does this sound possible? Toyota dealer said probably happened because I used independent repair shop with aftermarket cat converter & they don’t last. Has anyone else heard of this? Could it be anything else or do I have to pay another $550? Thanks.

The dealer is making statements for which he has no substantiating facts. It’s possible, but he doesn’t know.

The code P0420 is for, “Catalyst system efficiency below threshold (bank 1)”. This means that, from what the front and rear oxygen sensors are telling the engine computer, that the catalytic converter is doing a poor job. Are the oxygen sensors indicating correctly? It’s time to begin testing and troubleshooting.
This is where the proper use of a scan tool comes in. The scan tool is used to look at the values, and performance, of the oxygen sensors. From that look, the scan tool user can determine if the oxygen sensors, or the catalytic converter, is at fault. Unfortunately, too many repairers don’t use the scan tool fully and properly.

There is something else in the engine’s performance, or certain substances, which is adversely affecting the catalytic converter and the oxygen sensors. The repairer should attend to those, or else, the problem will come back. They are mentioned in the link, below.

Here is an article from a scan tool maker on the oxygen sensors and catalytic converters. It’s easy enough (I think) for even a non-mechanic to read, and learn from.