Engine Performance problem
1996 Geo Prizm 4 cyl Front Wheel Drive Automatic
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When the car reaches operating temperature a P0125 code comes up. Have replaced thermostat, flushed cooling system, replaced ECT sensor, and downstream o2 sensor. These were all the things my book suggested. The code still comes up. Any more suggestions?

Did you clear the code with a scan tool?

Yes, I cleared the code after each operation I performed. I took the car for emissions testing and it failed both because the light was on, then with a tailpipe test(High hydrocarbons and CO.

Check backpressure on the CAT. You may have a plugged CAT. Does it have an upstream sensor? If so it may need to be replaced, but you want to make sure the CAT is not clogged.

If it will not go into closed loop, it will run rich, which will clog the CAT.

I have already ordered a new upstream 02 sensor, not yet installed. How do I check for a clogged CAT?

You have to remove the upstream sensor and replace it with a special adapter and gauge, start the engine and measure pressure before the convertor.
You may be able to rent one.

You could also remove the downpipe from the convertor and inspect the convertor visually. It should look like a honeycomb, if you see cracks/melting the convertor must be replaced.