Nest · Furnace / HVAC
E74 No power to the heat-call (Rh) wire
What this code means
Nest's own documentation defines E74 as no power reaching the thermostat's Rh (heat call) wire — not a missing C-wire. This is almost always upstream at the furnace, not the thermostat wiring itself.
Most likely causes
- Tripped breaker or blown fuse on the furnace control board
- Failed or undersized furnace transformer
- Loose Rh wire at the furnace control board or thermostat base
What to try first
Check the furnace's own breaker and look for a small blown fuse on the furnace control board before assuming a C-wire problem.
Good news: this one is often fixable without a service call. If it persists after the steps above, have it looked at.
FAQ
What does Nest error code E74 mean?
No power to the heat-call (Rh) wire. Nest's own documentation defines E74 as no power reaching the thermostat's Rh (heat call) wire — not a missing C-wire. This is almost always upstream at the furnace, not the thermostat wiring itself.
Can I fix Nest code E74 myself?
Often, yes. Check the furnace's own breaker and look for a small blown fuse on the furnace control board before assuming a C-wire problem.
What causes Nest code E74?
The most common causes are: tripped breaker or blown fuse on the furnace control board; failed or undersized furnace transformer; loose rh wire at the furnace control board or thermostat base.
Other furnace brands: Trane · Carrier · Lennox · Goodman · Amana · American Standard · Rheem · Bryant · Generic
This information may contain errors or not apply to your exact model — always double-check it against your appliance's own manual before acting. If there's any chance of a gas, fire, electrical, carbon monoxide, or other safety hazard, stop and call a licensed professional or the appropriate agency (e.g. your gas utility) instead of relying on this page alone.