My new heat pump cooled fine in the summer but would not run on heat. At first it would trip on high pressure, then it would constantly trip on low pressure. Put the gauges on it and pressures were good in cooling but would drop to almost zero in heat mode.

Ended up being a bad heating side expansion valve. Mine is adjustable, and by fully exercising the adjustment screw, got it to work for about 30 minutes then same thing. Had to change it. My stupid 4 ton heat pump (top of the line 2 stage crap) takes 20 lbs of R410A!