Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
just a general question...can one use and INFRARED gun to determine which among the upstream and downstream exhaust is plugged instead of drilling holes per Scannerdanner method of drilling holes ?
Is the idea behind your question to detect that the cat outlet temperature would not be the typical 100 degrees F higher than the cat inlet temperature'? If so you might detect a completely clogged converter but I would not consider it a conclusive stand alone test.
Pulling an O2 sensor wouldn`t be my preferred testing method neither. Often times O2 sensors are placed in diffiicult to reach places, they are badly rusted and may break in the process or even the converter threads could strip.
A vacuum gauge reading of the intake manifold is a quick and dirty test ( on map sensor engines use this sensor), a secondary ignition test is almost fool proofed if you know what to look for. In-cylinder pressure captures will proof exhaust restriction as well.
When suspecting a clogged exhaust/cat causing a no start situation you may want to pull out a plug; the engine will fire up and at least you can pull it into the shop.
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
Well in that case, seeing as to how expensive cats are and not having to make a bad call on one, I think drilling holes would be appropriate in your case to take direct measurements.
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