Google Nest error codes, explained
Nest Learning Thermostat and Nest Thermostat. Codes below: E/H/W codes (E73, E74…) and power messages. Each guide covers what the code means, what you can safely try yourself, when to stop and call a technician, and what the repair typically costs.
Nest E73 — No power detected on the Rc wire
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.
Nest E74 — No power detected on the Rh wire
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.
Nest 'Delayed' message — Power-sharing (no C wire) problem
Nest shows 'Delayed for X min' when it can't draw enough consistent power. Without a C (common) wire, the Nest 'power-steals' from the heating/cooling circuits; when its battery runs low it delays calls for heat/cool to recharge — which reads as the office HVAC randomly ignoring the thermostat.