in heat pump/compressor technology. Before doing any of this, make sure you feel comfortable with working around this kind of stuff, especially in using a volt meter.

1. Is the outdoor fan running and the compressor not? If so you may have a compressor problem.
2. If neither is running check the fuses on the outside disconnect and the breakers in the breaker box. The power supply to the outdoor unit is separate than the indoor unit.
3. If both are good take the panel off the outdoor unit.
4. Check the contactor, is it pulled in? If not manually activate it and see if the unit comes on. If so you have a control (24V issue) or the contactor coil is bad.
5. Check for 24V on the coil of the contactor. If it has 24V the contactor is bad. If not you will have to check the 24V circuit back to the thermostat and figure out why you aren’t getting a call for the outdoor unit. (make sure your unit isn’t on emergency heat only on the thermostat.)
6. If the contactor is pulled in check the output side for voltage. If no voltage is present check the supply side. If there is voltage there the contactor is bad.
7. Some units will actually turn the outdoor unit off if the temperature drops to extremely low levels as you will actually be consuming more power than the heat being provided. If that is the case the outdoor unit will come back on when temps rise (probably >15°F).
8. If from #1 above the outdoor fan is running and the compressor not feel the compressor. If it is hot and you hear it hum occasionally then it’s locked up. If you are good enough with electrical circuits check the capacitor and make sure it is right and with an amp probe check starting currents. Beyond that you will need an HVAC guy to change out your compressor.
9. If you find your compressor is bad and the unit is old and uses R22 strongly consider replacing the whole unit with a new R410A unit.