E9
Nest
Heat Call Circuit Fault (W1)
Nest thermostat displays E9 and reports a problem with the W1 wire or heating system
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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 W1 wire at Nest base, Loose W1 wire at furnace control board, Blown low‑voltage fuse due to short on W1 to C, Shorted thermostat cable between thermostat and furnace
How to Fix / Troubleshooting
Safety: Turn off power to the furnace/air handler at the breaker before working on wiring.
- Check W1 at Nest: Remove the Nest display and verify the W1 wire is fully inserted and clamped.
- Inspect furnace board: Confirm the thermostat W1 conductor is attached securely to the W or W1 terminal on the control board.
- Fuse inspection: Check the low‑voltage fuse on the control board and replace if blown with the same amperage. If it blows again when heat is called, there is likely a short on the W1 circuit.
- Cable check: Inspect the thermostat cable from the furnace to the thermostat for staples driven too tightly, rodent damage, or sharp bends that may have cut the insulation.
- Test operation: After repairs, restore power and test a heat call from the Nest. If E9 persists, the furnace control board relay or Nest output driver may be faulty and require professional diagnosis.
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Repair Difficulty
Medium
3/5
Required Part
W1 Heat Call Wire / Low‑Voltage Cable to Furnace
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