Check engine light

I have a 2002 Subaru Outback with 134,000 miles. My check engine light came on and the mechanic said the code P042 indicated that the catalytic converter was bad. So they replaced it with an after-market part. Then the check engine light came on again after six days. The mechanic replaced the catalytic converter under warranty assuming it was just a bad part. He mentioned that if the light comes on again he suggest putting a OEM catalytic converter. The OEM would cost me an additional $450 on top of what I already paid. Could another part actually be causing the check engine light to come on and the catalytic converter is not the problem? Would an OEM part make a difference? Should I get a second opinion?

There is no check engine code that says the catalytic converter is bad, only some that say it might be.

P0420 is Catalyst System Efficiency Below Threshold Bank 1.

This could have been because of a bad converter, but I think we both know now that it probably wasn’t. It can also be caused by a failing o2 sensor (usually the rear one, but the front one is possible too), or an exhaust leak, or a few other possible problems.

You need to find a real mechanic, one that following the troubleshooting flowcharts instead of throwing parts at it over and over.