Why won't the heat pump turn on? The symptom narrows the next observation but does not identify a failed part. For the heat-pump system, first separate what the thermostat or normal controls request from what the indoor and outdoor equipment actually do. Note airflow, temperature change, timing, sound, odor, visible water or ice and whether the problem affects one room or the whole home, without opening a panel or reaching into equipment.
A trained technician can trace the thermostat request through power, safeties and controls, evaluate indoor and outdoor motors and electrical components, and test mode and defrost functions. Tell the service desk which sections respond and ask that diagnosis, electrical or control repair, heat-pump work, and confirmation in the requested mode be listed separately. Storm damage, utility outages, winter conditions, equipment access, permit rules, and electrician-versus-HVAC licensing boundaries depend on the address. Record thermostat power and mode, whether the indoor blower or backup heat responds, any outdoor sound or status light, and stop for smoke, burning odor, water at electrical equipment, or repeated trips. A completely inactive system, indoor-only operation, backup-only heat, and an outdoor unit that attempts to start are separate diagnostic states.