thermostat says "no power to heat pump" and my heat won't kick on, but the ac worked fine last summer

Diagnosis

The "no power to heat pump" error on an Amazon Smart Thermostat indicates a failure in the 24V AC control circuit between the thermostat base and the outdoor condensing unit. While your AC worked in the summer, the heating cycle requires a specific signal to the reversing valve (O/B wire) and the compressor. If the thermostat cannot detect the expected voltage return or if the circuit is open, it triggers this safety alert to prevent damage to the compressor. This most commonly happens because of a blown 3-amp or 5-amp fuse on the outdoor unit's control board, often caused by a power surge or a failing capacitor. Alternatively, it can be a "phantom" error caused by incorrect configuration of the reversing valve polarity in the software, where the thermostat attempts to engage a circuit that the hardware isn't responding to. You can confirm this by checking if the outdoor fan spins when the heat is called for; if the indoor blower runs but the outdoor unit is silent, the break is in the 24V communication line.

How to Fix It

To resolve the communication error and restore heating, follow these precise configuration and hardware steps:

1. Remove the thermostat faceplate from the wall. Inspect the O/B wire (usually orange or blue). Ensure the wire is stripped exactly 1/4 inch and is seated firmly in the terminal. If the wire is loose, tighten the terminal screw until the wire cannot be pulled out.
2. Reattach the faceplate and open the Alexa app. Navigate to Devices > Thermostat > Settings > Equipment Configuration.
3. Verify the system type is set to Heat Pump. If it is set to Conventional, the thermostat will not send power to the reversing valve, triggering the error.
4. Locate the Reversing Valve setting. This is the most critical step. If you have a Carrier, Bryant, or Trane system, set this to "O" (On Heat). If you have a Rheem or Ruud system, set this to "B" (On Cool). Selecting the wrong polarity will cause the unit to blow cold air when heat is requested, or trigger a power error.
5. Check the Auxiliary Heat setting. Ensure "Aux Heat" is enabled if you have electric heat strips in your air handler.
6. Cycle the power by flipping the HVAC breaker in your electrical panel OFF for 30 seconds, then back ON to reset the control board logic.

If That Didn't Work

• Check the Outdoor Fuse: Shut off all power to the HVAC system. Open the outdoor condensing unit's service panel. Locate the control board and look for a small automotive-style blade fuse (usually 3A or 5A). If the filament inside is broken, replace it with an identical amperage fuse. A blown fuse here is the #1 cause of "no power" errors when the AC worked previously but heat fails.
• Test for 24V Output: Using a multimeter set to AC voltage, place one probe on the C (Common) wire and the other on the O/B terminal at the thermostat base while the heat is calling. You should see approximately 24V. If you see 0V, there is a break in the thermostat wiring or a failed transformer.
• Inspect the Contactor: Check the outdoor contactor for signs of pitting or insects (like ants) getting stuck between the contact points. If the contactor cannot pull in, the heat pump will never receive power, regardless of the thermostat signal.
• Verify the Float Switch: Ensure your condensate drain line isn't clogged. Many systems have a float switch that cuts 24V power to the system to prevent flooding. If the switch is tripped, the thermostat will report a power loss.

Heads Up

• Avoid "Bridge" Wiring: Never jump the R wire to the O/B wire to "test" the system; this can blow the integrated circuit on the Amazon Smart Thermostat's baseplate.
• C-Wire Necessity: The Amazon Smart Thermostat requires a constant 24V C-wire. If you are using a "Power Extender Kit" or a "C-wire adapter," ensure the connections at the furnace control board are tight, as vibration can loosen these over time.
• Seasonal Testing: Always run your heat pump for 10 minutes in early autumn. Waiting until the first freeze to test the reversing valve often leads to discovering blown fuses when technicians are unavailable.
• Voltage Warning: When checking the outdoor unit, remember that the contactor handles 240V. Only touch the low-voltage (thin) wires unless you are using insulated probes and are trained in high-voltage safety.