Nest E73 — No power detected on the Rc wire

What this error means

The thermostat isn't receiving power on the Rc (cooling) wire. The classic hidden cause in offices: the AC's condensate drain line clogged, the float/safety switch tripped, and it cut power to the cooling circuit — the thermostat is the messenger, not the problem.

Symptoms

• E73 on the Nest display
• Cooling unavailable while heat may still work
• Often appears in humid months

Try this first (safe DIY steps)

1. Check the air handler's condensate drain pan and line — if the pan is full or the line is clogged, the float switch has likely cut power; clearing the clog (wet/dry vac on the drain line outlet) restores it
2. Check the furnace/air-handler service switch and the breaker for the outdoor unit
3. Pull the Nest display off its base and reseat it
4. Verify the Rc wire is seated in its connector (power off first)

Safety: power off before any physical intervention. Never bypass covers or interlocks — fuser areas run hot enough to burn.

When to call a technician

If the drain is clear and breakers are on but E73 persists — a blown low-voltage fuse on the air handler control board or a transformer issue needs an HVAC tech.