E73
Nest
Loss of 24V Power to Cooling Circuit (Rc)
Nest thermostat shows E73 and reports no power to the Rc wire (cooling 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
Tripped breaker or service switch for outdoor condenser, Blown low‑voltage fuse on furnace/air handler control board, Broken Y or Rc conductor between furnace and outdoor unit, Failed 24V transformer dedicated to cooling circuit
How to Fix / Troubleshooting
Safety: Turn off power to both indoor and outdoor units before opening panels.
- Check breakers: Verify that the AC condenser breaker and furnace/air handler breaker are on. Reset if tripped.
- Inspect low‑voltage fuse: At the furnace control board, check the low‑voltage fuse and replace if blown with the same rating.
- Measure Rc voltage: With power on, measure between Rc and C at the Nest base or furnace board. If there is no 24 VAC, trace back to the transformer and outdoor unit control circuit.
- Inspect condenser control wiring: At the outdoor unit, check the low‑voltage wires for breaks, corrosion, or rodent damage.
- Professional help: If the transformer or outdoor unit control board is suspected, contact an HVAC technician for replacement.
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Repair Difficulty
Hard
4/5
Required Part
Outdoor Condenser Control Circuit / 24V Transformer
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