Google Nest Error Codes & Diagnostics
Complete diagnostic directory for official Google Nest fault codes. Verify root causes, safe troubleshooting steps, and replacement part numbers before calling a technician.
Nest delayed messages: the thermostat is rationing its own battery
A Nest thermostat that says heating or cooling is delayed has detected that its internal battery is low and is postponing the call while it charges. Nothing needs to be wrong with the heating system at all. The thermostat is protecting its own ability to keep running, and the question worth asking is why it is not charging. Applies to: Google Nest learning and programmable thermostats that report power and delay conditions on screen This covers Google Nest thermostats that draw charge from the heating system wiring. Behaviour differs between Nest generations and between installations with and without a dedicated common conductor, so the Google support material for your specific model is the reference. Owner-safe checks: Note how often the delay message appears and whether it is worse in cold weather when the heating runs more. | Confirm that the heating itself works once the delay expires, which separates the control from the appliance. | Check whether a battery or power message has appeared alongside the delay at any point. Qualified technician: Running a new conductor to the thermostat position, or altering connections at a Heat Link or equipment terminal, is electrical work for a qualified person. | Do not add a conductor at the equipment end without knowing what it lands on. Heating control wiring carries mains voltage on many United Kingdom installations.
Nest E73: no power to the Rc wire, and the cooling side behind it
These codes report that the thermostat is no longer receiving 24 volt power on a named wire. Google groups E3, E23, and E73 as no power to the Rc wire, E4, E24, and E74 as no power to the Rh wire, and E195 as no power to the R wire, so the letter in the message identifies which transformer feed went quiet rather than which component failed. Applies to: Google Nest Learning Thermostat and Nest Thermostat This page is written around the Rc group of Nest help codes. HVAC Bench covers the Rh group, headed by E74, on its own page, and the two should not be read across: on a two-transformer installation they point at different equipment in different parts of the building. Whether your system has separate Rc and Rh feeds at all is a property of the installed equipment. Owner-safe checks: Check the breakers for the heating and the cooling equipment and the service switch beside the furnace or air handler. | Replace the air filter if it is dirty, which Google includes in its own troubleshooting sequence. | Look for water in the drip pan or a blocked drain tube under the indoor unit. | Turn off power to both the heating and cooling equipment before touching any thermostat wire. | With power off, confirm each thermostat wire is straight, clean, and inserted until the connector button stays pressed down. | With power off, check whether Rc and Rh were crossed if the thermostat was recently installed, and correct them if so. Qualified technician: A blown or blackened system fuse, whose cause needs to be found before replacement. | A condensate blockage that needs the drain line cleared. | An absent 24 volt supply once breakers, switches, and wiring have been confirmed correct. | Any equipment panel that would need to be opened to reach a transformer or a safety device. | Repeated recurrence of the same code after the drainage or filter has been corrected.
Nest E74: no power detected on the Rh wire
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.