Oxygen Sensors and Catalytic Converters

You have two converters. One per bank of cylinders. I included diagrams of this. Also, you have an upstream and downstream sensor for each converter for a total of 4 sensors.

Let us know what other info you need.

Doesn't the first diagram show a total of 3 catalytic converters? Two in the engine bay and one in the under-floor?

My mistake. You are correct. Normally the rear and the "under floor" are one unit but it appears they are all separate. The warm up converters (bank 1 and 2) are what the PCM monitors and uses to adjust air fuel ratios. The underfloor converter is just there are adding reduction in emissions. Notice how the underfloor does not have o2 sensors. That means it is not monitored so if you are having an issue with a code, the front two are going to be where the issue is.

In the event there is no check engine light and I wanted to diagnose if a catalytic converter is faulty, one of the test that can be performed is to use an infrared thermometer, and my understanding is that the temperature of the outlet catalytic converter is supposed to be higher than the inlet temperature. Is this correct? In the event that the temperature of the inlet is higher than the outlet, what would that tell us? Would that be a conclusive way of determining that the catalytic converter is faulty or would additional tests need to be carried out?

The temperature test works okay on the front converters, however if they are working properly there may not be enough of the correct gasses to ignite the rear unit so it could have a higher inlet than outlet temperature if that was the case. The two front units can be tested using the O2 sensors and the throttle. Just watch the live data from the upstream and downstream sensors and blip the throttle a few times. The front sensors will go rich and if the converters are working the rears may rise just a bit but if they track the fronts then you will want to test further.
This shows a few tests:
https://www.2carpros.com/articles/how-to-test-a-catalytic-converter