Have a XT1000, it'll run either 45 minutes or 3 hours (guessing 45 min checks every time) and sometimes it'll then frost, other times (most) it will turn off the compressor, the fan will keep running, but no defrost. If left alone if start back up after 45 minutes if it is left alone. What could be the problem? I suspect the defrost board...

Is ice starting to build up on the condenser coil(1 inch or so)? If so it may be just a simple defrost thermostat going bad. Not to rule out the logic in the control board going bad. If you have a lot of ice build up on the coil. It will be the defrost control board or defrost thermostat going bad.

No, I think it engages the defrost often enough since its never had ice buildup, just frost. I checked the defrost sensor plug into the board and its loose. I'm assuming this is most likely the problem.

If you have no ice build up on the condenser coil the defrost control board and defrost thermostat is working. Is it keeping the temp in the house of what you have it set on?

Right now the temp is set to 67 its 65 inside and the heat pump should be putting out air (warm), but its only room temp air coming out. The compressor is off, but the fan on the unit (outside) is running.

It'll do that for 45 minutes and then the compressor will start up again and put out heat. I have the aux off right now to diagnose the system a bit with a temp sensor at the closest vent

Other times it'll cycle the 45 minutes, go 45 more, then maybe a defrost, 45 min more running, then a 45 min cycle of nothing but room temp air. The system is set to auto fan and the fan on the unit is on so something is wrong. Tech found nothing

The defrost board will not shut off the compressor it shuts off the condenser fan motor. The compressor may be shuting off from over heating or a bad run capacitor.