Problem: heat pump produce no heat, only blow cold air.
Insight: Called for heat, Blower ran, but compressor didn't come on.
Cause: dirty air filter cause the high pressure switch to trip during heat mode.
Solution: While it was calling for heat, checked 24v on the Y wire from the thermostat at the unit, got 24v. checked 24v at the compressor contactor coil, no voltage. found High pressure swith is in between, reset it. cleaned the air filter, turned the heat back on.