E74
Nest
Open Cooling Call Circuit (Y1 to Condenser)
Nest thermostat displays E74 and reports no power to the Y1 wire (cooling call)
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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
Broken Y1 conductor between furnace and outdoor unit, Loose Y1 connection at furnace control board, Loose or corroded Y connection at outdoor condenser contactor, Blown low‑voltage fuse due to previous short on Y1
How to Fix / Troubleshooting
Safety: Turn off power to indoor and outdoor units before working on wiring.
- Check Y1 at furnace: Confirm the thermostat Y1 conductor and the outdoor unit Y conductor are both attached to the Y terminal on the furnace/air handler control board.
- Inspect outdoor connections: At the condenser, open the control panel and verify the low‑voltage wires are firmly attached to the contactor coil terminals.
- Cable continuity: With power off, test continuity of the Y1 conductor from furnace to outdoor unit. Replace cable if open.
- Fuse check: Inspect and replace the low‑voltage fuse on the furnace board if blown.
- Test call: After repairs, restore power and call for cooling from the Nest. If E74 persists, the contactor coil or control board may be faulty.
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Repair Difficulty
Hard
4/5
Required Part
Y1 Cooling Call Wire / Outdoor Condenser Contactor Circuit
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