I’ve replaced the upstream 02 sensor, cleaned the catalytic converter and I had 1 mechanic (come by the house) say that everything checks out good but the codes keep popping up.

I haven’t gone to a shop yet but I’m just wondering what it could be. We cleaned the catalytic but it can very well still be it. Other things it may be, 02 sensor wire issue, a bad electronic control unit, anything else to consider?

Any advice or what I should say to the mechanics at the shop? Obviously don’t want to spend on unnecessary parts.

I failed my smog check with the following:

1. OBD2 / OBDII
2. OBD Bulb Check
3. P0421 - Catalyst 1 Efficiency Below Threshold Bank 1