The best simple test for cat restriction I have heard is to put a vacuum gauge on the intake. Look at the numbers at idle and then run the speed to 2000rpms and hold it there. The vacuum at 2000 (unloaded) should be greater than at idle. If it is restricted it won't be.

If you can barely get to 70 then you could have a cat problem, but check it with the vacuum gauge.

As to the temp means of testing. One must use a infrared temp tool to make such judgements. If you can touch and hold any part of the cat it isn't hot enough to work anyway. Drive it down the road for ten minutes and then do the test with the infrared gun.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician