Nest Thermostat No-Power Codes (E23, E24, E72, E73): Meaning & Fixes

Nest's no-power codes — E23, E24, E72 and E73, with E74 covered on its own page — all report the same event from different wires and different thermostat generations: power that should arrive at the base on an R, Rc or Rh wire is not arriving. This guide splits the four by wire, walks the shared power path they all depend on (supply, switch, connection, equipment), covers the legacy E3 and E4 readings honestly, and ends where Google's own flow ends — with the summer-breakdown pattern that makes these codes a professional's job. Read your code's two coordinates first — which wire, which generation — because they decide everything that follows, from which breaker you check to which system's transformer feed you suspect.

How the four codes split: by wire first, by generation second

Two questions decode any code in this family, and the wire question comes first. A Nest base with separate Rc and Rh connectors is fed by two power paths: Rc carries the cooling system's power and Rh carries the heating system's power, and the thermostat reports each path separately — no power to Rc is one report, no power to Rh is another, and no power wires detected at all is a third, wider one. A base with a single R connector cannot split the report, so the same losses collapse into E195, 'No power to R wire detected', covered on its own page in this site's Nest section. The second question is generation, because Google re-numbers the same reports as the thermostat line evolves: the master help-code table groups E3, E23 and E73 together as one meaning, and E4, E24 and E74 together as another, while E72 is grouped with E2, E22 and E402 as 'No power wires detected — Rc or Rh wire required'. Same fault, three numberings. That is why chasing the exact number matters less than reading its two coordinates: which wire lost power, and which generation is reporting it. Every code in this guide is an E code, and Google's rule for the family sets the stakes before any troubleshooting starts: E codes are critical, must be fixed as soon as possible, the thermostat won't allow you to control heating or cooling, and the message won't go away until the cause has been fixed. There is no dismiss-and-monitor path for a no-power code — the only way out is power, restored and detected. Keep those two coordinates written down beside the thermostat while you work; every check in the sections that follow is addressed to one of them, and a check aimed at the wrong wire wastes an afternoon and changes nothing.

E23 — No power to Rc wire detected (the earlier-generation Rc report)

Google groups E23 with E3 and E73 under one definition: 'No power to Rc wire detected', routed to Troubleshoot power. The Rc wire is the cooling side's power feed, so an E23 says the thermostat can account for its wiring but the cooling system is sending no power down the wire that should carry it. Google's power flow gives the causes their order. First, the supply: make sure the power to the system is on — breaker box or fuse box, or the switch that looks like a light switch near the equipment — remembering there may be two switches, one for heating and one for cooling, and Rc answers to the cooling one. A cooling breaker left off after maintenance, or a fuse gone in the fuse box, produces a textbook E23 on a system with nothing else wrong. Second, the connection: with power turned off at the breaker or fuse box, pull the display and check the Rc wire is fully inserted, its connector button stays down, and it carries clean exposed copper — Google's checks specify 1/3 to 1/2 inch of straight copper in its detailed flows. Third, the equipment itself: Google's No-power article names the catch-all honestly — the HVAC system experiencing some other issue that prevents it from sending power to the thermostat wires — and its hot-weather section sharpens it for summer arrivals, when the cooling system is the one working hardest and failing most. An E23 that arrives on the first hot week, on a system that cooled fine last year, is Google's breakdown pattern, not a wiring coincidence, and its flow ends at an HVAC professional after the supply and seating checks.

E73 — No power to Rc wire detected (the same loss, the later base)

E73 carries the identical Google definition as E23 — 'No power to Rc wire detected', grouped with E3 and E23 — because it is the identical fault reported by a later thermostat generation. Nothing about the cooling system changes between an E23 house and an E73 house; only the number on the screen does, which is why this guide treats them as one diagnosis with two labels and why quoting the code alone, without the model, can send a technician to the wrong table. Work an E73 exactly as an E23: cooling-side supply first (breaker, fuse, and the cooling system switch if your home has separate switches), then the Rc connection with power off (fully inserted, connector button staying down, clean straight copper), then the equipment. Two E73-specific habits are worth forming. First, check whether the heating side still has power: on a split Rc/Rh base, an E73 with heating functions alive localises the loss to the cooling feed — its supply, its transformer path, its equipment — while an E73 accompanied by dead heating usually means the wider report, E72, is the truer description of a shared supply failure. Second, respect the season signal Google builds into its guidance: it names E73 explicitly in the summer list (E3, E4, E23, E24, E73, E74, E195) for which a hot-weather arrival on a previously working system likely means the HVAC system has broken down and stopped powering the thermostat — with the added warning that if the problem persists, the thermostat's battery will drain and it may disconnect from the internet or shut down. Charge a dying display by USB while you arrange the visit; do not let the messenger die before the technician reads it.

E24 — No power to Rh wire detected (the earlier-generation Rh report)

E24 is E23's mirror image: Google groups it with E4 and E74 under 'No power to Rh wire detected', routed to Troubleshoot power. Rh is the heating side's power feed, so every E23 check translates one system over. The supply question becomes the heating system's breaker, fuse and switch — and split-switch homes are where E24 earns its keep, because a heating switch left off after a summer of cooling-only running announces itself exactly this way when the first cold evening arrives. The connection question is the Rh wire's seating, checked with power off: fully inserted, button staying down, clean copper. The equipment question is whether the heating system has stopped sending power — Google's umbrella cause — and here the mirror-image season logic applies: Google's named summer-breakdown pattern is written around cooling, but a heating system that fails under its first sustained load of winter shuts down and stops feeding Rh in the same way, and the thermostat reports what it sees either way. One check belongs to E24 and E74 alone, and Google states it in its overcurrent troubleshooting for exactly this pair of connectors: if you have wires in both the Rc and Rh connectors, double-check they weren't accidentally switched during installation, and swap them if they were. A crossed pair can leave Rh expecting power from the cooling feed — an E24 that survives every breaker check on a recently installed thermostat is a wiring-diagram problem: compare the base, wire by wire, against the diagram the app gave you during setup before suspecting the furnace. That comparison costs ten minutes with the power off, and on a recently installed or recently serviced system it is statistically where this code lives — in the gap between the wiring the system has and the wiring the thermostat was told about.

E72 — No power wires detected: Rc or Rh wire required

E72 widens the lens: Google groups it with E2, E22 and E402 under 'No power wires detected — Rc or Rh wire required', routed to Troubleshoot power. Where E23, E24, E73 and E74 each report one feed dead, E72 reports that the thermostat can find no power wire delivering anything — either no R-family wire is properly connected at all, or the shared supply behind both feeds is dead, so neither Rc nor Rh has power to report. The diagnosis therefore starts one level higher than the single-wire codes. Supply first, and completely: breaker and fuse for the whole system, and both system switches if there are two — an E72 after electrical work, a storm or a service visit is a house-supply story until proven otherwise. Then, with power off, the base itself: is there actually a wire in Rc or Rh, fully seated with its connector button down, and is it the right wire — compare against the app's wiring diagram, because a thermostat installed or re-wired without its power wire landed correctly will report E72 faithfully forever. E72 is also the likeliest code in this guide to be an install-time report rather than a mid-season failure: a system type that Google names as requiring a C wire (its No-power article lists heat-only, cool-only, zone-controlled and heat pump systems) can leave a new thermostat without a workable power path from day one. Distinguish the two arrivals and the fix chooses itself — a mid-season E72 is a supply or equipment failure hunt, an install-time E72 is a wiring and compatibility job, and both end at a professional if the supply and seating checks come back clean.

The shared power path: supply, transformer feed, fuse, drain and the C wire

All four codes — and E74 and E195 beside them — are reports from one chain, and walking the chain in order is faster than walking the codes. The chain starts at the house supply: breaker box or fuse box, then the system switch (or switches) near the equipment. Next, the equipment itself: the HVAC system generates the low-voltage power the thermostat lives on and sends it up the R-family wires, which is why Google's umbrella cause for this whole family is the system stopping sending power — a system shut down by its own protections sends nothing, whatever the breakers say. Google's hot-weather steps for these codes name the protective shutdowns it wants checked, in its order: the air filter (a clogged filter limits airflow, cooling coils can freeze solid, and the system shuts off), then the drip pan and drain tubes (backed-up water makes the system shut off to prevent overflow — and Google names the float switch, also called a drain switch, fitted on the drain tube by a thin wire, as the device that does it, to be lifted and checked for water where one is fitted), then the HVAC fuse on the control board (more likely to burn out when the system works extra hard; inspect for blackening, discoloration or a broken filament, with the new fuse labelled, often a small U-shaped '3' fuse). Those equipment steps are headed by Google's high-voltage warning and are professional territory for most owners. Last in the chain sits the C wire, and its place needs precision: a C wire gives the thermostat a complete, stable power circuit, and Google requires one (or a Nest Power Connector) on heat-only, cool-only, zone-controlled and heat pump systems — but a C wire completes a circuit, it does not create the supply. Where a feed has died, the C-wire codes (E1, E297, N260) are a different family, and fitting a C wire never resurrects a dead Rc or Rh feed on its own.

Legacy E3 and E4: what Google actually documents

Short answer: more than the rumour suggests, and less than a standalone article. Google's master help-code table lists E3 in its first-generation column and documents it by group: 'E3, E23, and E73 — No power to Rc wire detected', and likewise 'E4, E24, and E74 — No power to Rh wire detected'. Google's hot-weather section uses the same grouping in prose, naming E3 and E4 among the codes a summer system breakdown produces. So E3 and E4 do have published Google meanings — they are the first-generation Nest Learning Thermostat's renderings of the two split-power losses, E3 for the Rc feed and E4 for the Rh feed, with the same causes, the same Troubleshoot power flow and the same professional endpoint as their later-numbered equivalents. What Google does not publish is anything beyond that: no dedicated E3 or E4 article, no E3-specific causes, and no separate procedure — which is why this guide gives them a shared paragraph rather than invented per-code pages. The practical consequence for an owner of a first-generation thermostat is simple: translate, then follow the modern number. An E3 is worked as an E73 (cooling-side supply, Rc seating, equipment), an E4 as an E74 (heating-side supply, Rh seating, and the Rc/Rh swap check), and if a search result or a forum post assigns E3 or E4 some other, more exotic meaning — a sensor fault, a software state, a reset procedure — it is not drawing on Google's documentation, because Google's documentation contains nothing else for these two codes. Quote both numbers when you call for help: 'E4 on a first-generation Learning Thermostat — the Rh no-power code' is a complete, verifiable diagnosis.

Battery drain, summer breakdowns and when to stop and call a pro

Three closing patterns govern every code in this guide. The first is time: a Nest thermostat on a dead feed lives on its battery, and Google warns that if a no-power problem persists, the battery will drain and the thermostat may disconnect from the internet or shut down entirely — a blank thermostat a day after an E73 is the same fault, one stage later, not a new one. Keep the display alive as Google directs (charge it with a USB cable for at least 10 minutes in its hot-weather steps) so the code and the wiring screens remain readable. The second pattern is season: Google's hot-weather guidance exists because air conditioners fail more frequently in hot months, and it names this guide's whole code set as the signature of a system that broke down and stopped powering its thermostat. A code that arrives in a heatwave on a system that was working should short-circuit your troubleshooting: breakers checked, then a professional booked, with the filter, drip-pan and fuse steps left to that visit unless you are equipped for Google's advanced, high-voltage-warned steps yourself. The third pattern is the endpoint Google itself sets: its Troubleshoot power flow closes, after the supply and wiring checks, with 'contact a local professional' — so a code still standing after the supply is proven on, both switches are on, the named wire is reseated with clean copper and its button down, and Rc/Rh are proven un-swapped, is not a failed DIY attempt. It is the diagnosis completing itself: the equipment has stopped sending power for an internal reason, and the thermostat has already told you everything it knows.

Key takeaways

• Translate before you troubleshoot: wire first (Rc = cooling feed, Rh = heating feed, E72 = neither), generation second — E3=E23=E73 and E4=E24=E74 are one fault each in three numberings.
• Summer arrival on a working system is Google's breakdown pattern: breakers on, then book the pro — and charge the display by USB so the battery doesn't drain and take the evidence with it.
• Rc and Rh both wired? Check they weren't swapped at installation before suspecting either system — Google names that exact check for this pair of connectors.

Resetting the Nest thermostat

Nest help codes don't clear like appliance fault codes. Google states the rule for E codes plainly: they are critical, the thermostat won't allow you to control heating or cooling, and the message won't go away until the cause has been fixed — so the 'reset' for an E code is repairing its cause (power restored, wire reseated, configuration corrected) and letting the thermostat re-detect a healthy system. N codes are the opposite kind of message: if the system is running normally, Google's low-power guidance says you can dismiss the message and keep using the thermostat. Two genuine resets exist at the edges: a manual restart (on Nest Thermostat E and 3rd-gen Learning Thermostat, press and hold the ring for about 10 seconds until the screen turns off — Google reserves this for a stuck or frozen thermostat), and a factory reset, which Google's wire-configuration flow (the E294 group) uses deliberately: reset to factory defaults, add the thermostat back in the Google Home app, and re-enter the wiring info carefully. Neither reset repairs a standing power or wiring fault.

Frequently asked questions

Related guides

Facts on this page were checked against manufacturer documentation: support.google.com · support.google.com · support.google.com · support.google.com · support.google.com · support.google.com · support.google.com · support.google.com · support.google.com · support.google.com · support.google.com · support.google.com · support.google.com