Surging engine - Any ideas???

the tech not seeing any codes for the cat being bad kind of makes sense... I dont think you gave it enought time to generate a CEL, if any. There is no check for a plugged cat, just one that doesnt function right.

 

Well there's an 02 sensor before the cat and an 02 sensor after the cat which measures the difference in airflow through the cat. The difference in flow can be enough to trigger a CEL.

Also when diagnosing on some other vehicles, you can run a vaccum gauge on the cat to test its effectiveness.

I've always used a IR tempature gun to check the temps.