Nest E74: no power detected on the Rh wire

What Does Error E74 Mean?

Google Nest defines E74 as no power detected on the Rh wire. The thermostat is reporting the missing 24-volt heating-power input, not its cause. Furnace power, transformer and fuse, equipment safeties, condensate devices, Rh wiring and connection, and the thermostat input remain separate diagnostic points. Applies to: Google Nest thermostats covered by the E-code power guide This E74 definition applies to Nest models covered by Google's wiring-error guide. The 2020 Nest Thermostat can present different messages, and Rc, C, and other E-codes identify different inputs. Owner-safe checks: Save the E74 and wiring screen, identify whether equipment also lost power, and switch HVAC power off before any manual-approved connector inspection. Qualified technician: Transformer, fuse, equipment safety, condensate control, concealed wiring, voltage measurement, and thermostat-input diagnosis require qualified HVAC service.