Try DIY first — may need a proVerified by FixOffice
Nest E74 — No power detected on the Rh wire
Applies to: Nest Learning Thermostat, Nest Thermostat
What this error means
No power on the Rh (heating) wire — the heating system isn't delivering 24V to the thermostat. Common causes: tripped furnace switch or breaker, a blown low-voltage fuse on the furnace control board, or a condensate overflow switch on high-efficiency furnaces.
Symptoms
- E74 on display
- Heating unavailable
- May follow a power outage or furnace service
Try this first (safe DIY steps)
- Check the furnace power switch (looks like a light switch near the unit) and the breaker
- If your furnace has a condensate pump/overflow switch, check it isn't tripped or full
- Reseat the Nest display on its base
- With power off, confirm the Rh wire is firmly seated
Safety: power off before any physical intervention. Never bypass covers or interlocks — fuser areas run hot enough to burn.
When to call a technician
Persistent E74 with power confirmed usually means a blown 3A/5A fuse or transformer issue on the furnace board — quick fix for a tech, and worth asking them why it blew.