Worked on a Lennox two speed heat pump HP21. customer complained that it would run for an hour or two and then quit cooling. Gave a general once over, running at 70psi low side, under 200 high side, outdoor was 82 degrees F.

After it ran for 60 min, the low side would gfadually drop to 0 psi and even into the HG zone a little. It never freezes up the indoor coil, not much freon flowing, low side not cold to reversing valve.

From the txv equalizer bypass the line would freeze up down to the low side line and give the compressor some cooling, but not much, a little bit of cold r22 seemed to be flowing through the txv. I did the magnet trick on the receiver check valve that is in the outdoor unit right below the txv, turned it all off and ran the test on the TSC, a code 42 came up, which is a hot windings or ruined compressor sensors. Checked the sensor ohms, and running dc voltage on the TSC terminals for the Compressor sensor, all was well and I suspect it may have been an earlier recall code due to the low side pressure dropping, the volume drops too, as the low side line coming back to the reversing valve is not cold and returning any refrigerant. The high side never changes, not a bit, always holding below 200.

Started it all back up, same problem, only sometimes it is a gradual decline in low side pressure, always to 0, sometimes it happens faster than usual, but back to 0. Tested reversing valve, temperatures are equal from return to compressor, very hot coming from compressor to reversing valve, as one would expect energized in cooling mode.

I'm racking me brains on this one, do I have a back check valve in the outdoor unit or txv? When put in the heat mode, it all runs fine. I have several Lennox and heat pump books, but can't find the solution.

Thanks, Mike