A/C Freeze Sensor - How do I check a faulty sensor?

We only have one a/c. We have checked the separation gasket and sealed between the intake and exhaust. If we remove the Thermister from the coil, the compressor starts and shuts off after exactly 2 minutes. We can try disconnecting the plug from the board and see what happens. Once the a/c runs for longer than 2 minutes, most of the time it's good until we shut it down.