E12
Amazon Smart Thermostat
Reversing Valve (O/B) Circuit Fault
Thermostat shows E12 and heat pump will not heat or cool correctly
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Miswired O/B terminal, Shorted O/B wire, Incorrect O/B setting in configuration, Failed O/B output driver on thermostat
How to Fix / Troubleshooting
Safety: Turn off power to both indoor and outdoor units at the breaker.
- Confirm system type: Verify you have a heat pump system (not conventional furnace + AC). Heat pumps require correct O/B wiring.
- Check O/B wiring: Ensure the reversing valve wire from the outdoor unit is connected to the O/B terminal on the thermostat wall plate and to the correct terminal on the air handler/furnace control board.
- Verify O/B setting: In the Alexa app, confirm whether your heat pump uses O (energize in cooling) or B (energize in heating). Set this according to the outdoor unit manufacturer’s documentation.
- Inspect for shorts: Look for damaged insulation on the O/B wire along its path.
If E12 persists and the heat pump does not switch modes correctly, the thermostat’s O/B driver may be defective and the thermostat should be replaced.
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Repair Difficulty
Hard
4/5
Required Part
Amazon Smart Thermostat Main Control Assembly
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