E12
Nest
Heat Pump Reversing Valve Control Fault (O/B)
Nest thermostat shows E12 and reports a problem with the O/B wire or heat pump reversing valve
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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
Loose or disconnected O/B wire at Nest base, Loose O/B wire at air handler or outdoor unit, Misconfigured heat pump O/B setting in Nest, Shorted O/B conductor in thermostat cable
How to Fix / Troubleshooting
Safety: Turn off power to both indoor and outdoor units before working on wiring.
- Check O/B at Nest: Remove the Nest display and verify the O/B wire is fully inserted and clamped.
- Inspect air handler/outdoor unit: Confirm the same conductor is attached to the reversing valve terminal (often labeled O or B) at the air handler or outdoor unit control board.
- Configure Nest: In the Nest settings, ensure the system is set as a heat pump and that the O/B orientation (energize in heating vs cooling) matches the heat pump manufacturer’s specification.
- Cable inspection: Inspect the thermostat cable for damage that could short O/B to C or another conductor.
- Test operation: After corrections, restore power and test both heating and cooling. If E12 persists or the system heats when it should cool (or vice versa), a technician should verify the reversing valve coil and control board.
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Repair Difficulty
Hard
4/5
Required Part
O/B Reversing Valve Control Wire
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