This refrigerator is not cooling...the freezer or fresh food compartment. Fan is constantly running in the freezer compartment. Putting out some cool air but not cold enough to get to zero degrees. I tested the compressor pins and if I got the correct pins then my start pins show 4.5 ohms, run pins show 3.0 ohms (could be backward) and run to start shows 7.0 ohms. I also checked ptc start relay reading (no rattling sound if shook) and got 81.8 between top pin and one of side by side pins, got 75.7 ohms on the other pin with the top pin and got 6.4 ohms between the two side by side pins. I am thinking that the start relay is bad...I heard some of the same clicking that others have described.. and would really like to have some advice before I order the start relay.

Here are your parts
Parts for Frigidaire FRT21INLHW3 Refrigerator - AppliancePartsPros.com

It sounds like your compressor is overheating.
You did not say but I am assuming the the compressor is on all the time (except if it went through a defrost cycle) and the unit never did start cycling correctly before it failed.

You said you only get cool air so I would check that out first.

Let the compressor cool down again .
Then restart it, let it run for half an hour or so and then remove the cover in the freezer so you can inspect the evaporator coils.
They should have a fairly even coating of frost on them.
If there is just frost in one area usually where the freon enters the coils then you have a sealed system problem. This will be an expensive repair as you need a pro and often it makes more sense to put the dollars towards a new unit.

If you find that the coils are clogged solid with ice/frost then it is a defrost problem and is fixable for a reasonable cost.

My premise on both the above is that the compressor cannot run at 100 percent duty cycle and is just overheating because it is on for too long.

Could also b as you suggest that the starter is bad. Note that there is not a good way to troubleshoot a PTC starter so it is take a gamble and try a new one.

I started it back up and after about 30 minutes or so I removed the back cover in the freezer, and the coils are completely clean and free of any frost or ice. And at one point I heard the compressor trying to come on but not doing it. Not knowing the electrical circuit and assuming the humming I heard was the ptc start relay, could that be the defrost control?