Heat Pump Troubleshooting, At the outdoor unit
If you have decided the problem is with the outdoor unit, return the thermostat to normal from emergency heat. Wait a couple of minutes, then continue your heat pump troubleshooting at the outdoor unit.
Is there a lot of ice/frost buildup on the unit? If there is, The problem is either a defrost control or the unit could be low on refrigerant. You can test the defrost control by manually forcing it to defrost. On older units, they had a defrost timer motor and you could manually advance the timer motor to initiate defrost. If you do so and the unit does not go into defrost, the timer motor is probably bad. On newer units, they have an electronic defrost control board. They typically have a set of pins that are jumped to manually initiate defrost. If you do so and the unit does not enter defrost mode, the board is probably bad.
Is the outdoor fan running? If it is not and you live in a colder area, the unit may have an outdoor thermostat installed. If it is colder than the setpoint the unit will not run. Otherwise, you should continue your heat pump troubleshooting by checking the high pressure cutout, for a wiring problem, the fan motor, or the run capacitor.
If the fan is running, feel the refrigerant lines. The bigger line should be warm. If it is not, the problem could be a wire in the unit, the compressor run capacitor, compressor, or the unit could be low on refrigerant.
**NOTE** It is best to check the charge of a heat pump with the unit running in the cooling mode. If possible, wait until the outdoor temperature is above 55 degrees.