Nest Thermostat Error Codes List: Every E, N & TD Code Explained

Every Nest thermostat error code we could verify against Nest's own documentation — what it means, what usually causes it, and the first fix to try — in one table. Codes differ between model series, so treat your model's tech sheet as the final word.

How to read and retrieve error codes on Nest thermostats

Nest surfaces help codes in two places: as a message with a code on the thermostat's own display, and in the Nest or Google Home app (TD codes, for example, appear in the app during setup, sometimes paired with an m15 message on the thermostat screen). The diagnostic screen that matters most is Settings > Equipment > Wiring info: it draws every connected wire, and a wire that isn't sending voltage shows grey — on a no-power alert, all-grey wires point at the system supply, a single grey wire at that wire's connection. Settings > Equipment also hosts the system Test mode that runs heat, fan and cooling on demand (and the Furnace Heads-Up test on equipped models). History lives in the same menus — Quick View > History on Learning Thermostats, Settings > History on Thermostat E — as Energy History rather than a fault log, so note a code down (or photograph the display) before it clears: once power returns, an E code leaves no trace in the history screens an owner can open.

The complete Nest thermostat error code list

How to reset / clear error codes on Nest thermostats

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.

How to diagnose any Nest thermostat code

Work the problem in this order: (1) write the code down exactly, including its letter prefix — on a smart thermostat the prefix names the family (a power report, a wiring report, a connection report), and the family matters more than the number; (2) remember what the thermostat is: a witness, not the patient. Most thermostat codes report the heating/cooling equipment — a furnace that lost power, a float switch that tripped, a wire that is not seated — so the second stop is always the equipment itself: is its power switch on, its breaker untripped, its access panel fully closed (the panel switch kills power on many furnaces), its condensate drain clear; (3) check the wiring at the thermostat base with the equipment powered OFF — a wire pushed beside its terminal instead of into it is the classic cause of wire-not-detected codes; (4) restore power and let the thermostat re-read the system before judging the result, because power codes clear only when the equipment actually delivers power again; (5) if the equipment runs normally from its own test mode but the thermostat still reports the fault, the wiring run or the thermostat itself is the remaining suspect — and 24V wiring faults inside walls or at the equipment board are a technician's job.

The help-code families: E, N, TD and M — and why most E codes are about your HVAC system, not your thermostat

Google calls these 'help codes', and the letter is the first diagnosis. E codes are the critical family: in Google's words, an E code must be fixed as soon as possible, the thermostat won't allow you to control heating or cooling while one is showing, and the message won't go away until the cause has been fixed. Read the E table and a pattern jumps out — almost none of it describes a broken thermostat. The biggest E blocks are the no-power codes (E72/E73/E74 and their Rc/Rh siblings, E195, E298/E448), the wire-not-detected codes (E157, E158, E184, E194), the overcurrent group (E52-E58, E102-E108 and relatives) and the Y1-power group (E10, E30, E66, E80, E116, E160, E196). Every one of those is the thermostat measuring something about the wires and the equipment behind them: power that stopped arriving, a wire that fails its detection test, current that spiked repeatedly on a wire. The thermostat is the most instrumented device in the system — it tests its wires mechanically and electrically, watches current on them many times a second, and lives or dies by the power the equipment sends it — so it is usually the first thing in the house that knows the furnace or air conditioner has a problem, and the E code is its report, not its failure. N codes are the non-critical counterpart — Google literally files them as notices alongside the E errors: low power on a wire (a wire receiving power, but less than expected), or a recommendation such as N260/N410, 'a C wire or Nest Power Connector is recommended'. Where an E code blocks control, Google's low-power guidance says an N message can be dismissed if the system is running normally — it is advice with a code attached, not an alarm. TD codes leave the HVAC system entirely: they are the connection family, shown in the app when the thermostat has problems connecting to Wi-Fi or to the app, or when adding it to an account fails — Bluetooth pairing to the phone, a Thread assisting device too far away, a Wi-Fi network change. Google publishes no expansion of the letters 'TD'; functionally, TD is the setup-and-connection drawer, and a TD code never means a heating fault. M codes are the miscellaneous drawer: Google's M table is mostly a set of doors into help topics (M6 routes to Furnace Heads-Up, M20 to the No-power article, M27 to the Nest Power Connector), and only some M codes carry fault meanings of their own in the troubleshooting flows — M6 (system needs maintenance), M13 (the Y1-power family), M15 (the thermostat couldn't be added to your account) and M20 (no power). One practical consequence follows from all four families: quote the whole code, letter included, when you search or call for help. E74, N74-renderings and TD codes are different reports from different drawers, and the third-party tables that strip the letters or invent neat E1-to-E5 lists are exactly how wrong diagnoses start.

The C wire and the power story: why no-C-wire installs produce the no-power family, and what the Nest Power Connector is

A Nest thermostat is a computer that lives on a wire designed for a switch, and most of Nest's code families are that tension surfacing. The C (common) wire, as Google's compatibility guide puts it, doesn't control heating or cooling — it helps deliver power from the system to the thermostat. Nest thermostats were engineered to use as little power as possible, so in many cases they don't need one: without a C wire, a Nest can power itself through the other wires — Google's Y-wire article explains the mechanism for the commonest case, where the thermostat charges itself through the Y1 (air conditioner) wire while the system sits idle. That arrangement works until the wire it borrows stops being available. If the air conditioner trips a cold-weather safety cut-out, or breaks down, or simply stops sending power, the thermostat loses its supply and files the report it has: the Y1-power group (E10, E30, E66, E80, E116, E160, E196 and M13), the low-power N notices when the borrowed power merely sags below expectation, and the battery symptoms Google lists for a starved thermostat — battery keeps draining, repeated Wi-Fi disconnects, features such as Farsight, wake-on display and occupancy detection switching themselves off. Some systems can't lend power safely at all: Google's No-power article names heat-only, cool-only, zone-controlled and heat pump systems as types that require a C wire or a Power Connector, and on those systems a no-C-wire install doesn't degrade gracefully — it produces the critical no-power codes (E1, E297, E298/E448, E195's C-wire branch) from day one. The permanent fix is to give the thermostat its own supply. If a spare wire already runs from the thermostat to the equipment, it can usually be connected as a C wire at both ends. If not, the choices are running a new C wire — a pro job that can be expensive through finished walls — or Google's own accessory, the Nest Power Connector: a small module installed at the furnace or air handler (at the zone controller on a zoned system) that provides a stable power source for the thermostat when the system can't deliver enough consistent power over its existing wires, without running a new wire to the thermostat. It works with the Nest Thermostat, Nest Thermostat E and the 3rd- and 4th-gen Learning Thermostats, on 24 VAC systems only, and Google warns against third-party C-wire adaptors, which can cause problems or damage the thermostat. The codes are the tell for when you've reached that point: E1, E297, N260, N261 (and their 4th-gen partners N410, N411) plus M27 are Google's C-wire group, and a thermostat that keeps filing them after its wires have been reseated is not asking for another reset — it is asking for a C wire or a Power Connector, fitted once, after which the whole borrowed-power family of codes goes quiet.

The most searched Nest thermostat codes, in detail

E195 — No power to R wire detected

E195 is the single-R rendering of Nest's no-power family. Models with separate Rc and Rh connectors report the same loss as E73 (no power to Rc) or E74 (no power to Rh); a thermostat with one R connector reports E195 — the R wire is present, but no power is arriving on it. That distinction matters: E194 ('R wire not detected') is the detection failure, E195 is a power failure on a wire the thermostat can see. And like every E code, Google classifies E195 as critical: the thermostat won't let you control heating or cooling, and the message won't go away until the cause is fixed. In most cases the thermostat itself is the messenger, not the fault — the equipment that feeds the R wire has stopped feeding it, whether because its power is off, its cover is open, or the system itself has shut down or broken down. Google's hot-weather section singles this code out: in summer, an E195 (with E3, E4, E23, E24, E73, E74) usually means the HVAC system has broken down and stopped powering the thermostat, and if it persists the thermostat's battery drains until it disconnects from the internet or shuts down entirely.

Most likely cause: System power off at the breaker, fuse box or system switch — Google's No-power article lists this first among its causes, and its hot-weather section adds the classic variant: breakers left off after construction or maintenance. Its thermostat-won't-turn-on article adds the fuse-box version — check the fuse controlling power to the system isn't blown. Nothing downstream of a dead supply can power the R wire.

First check: Confirm the system is powered: breaker on in the breaker box, fuse intact in the fuse box, and the system switch (often a light-switch-like switch near the equipment — there may be one for heating and one for cooling) on. If you recently had construction or maintenance done, this is Google's named first check.

If it keeps coming back: E195 returning after a reseat and a proven supply means the equipment is cutting power intermittently: a system shutting itself down as it runs (the clogged-filter/overheating pattern Google describes for Furnace Heads-Up), a cover switch that loses contact, or a system in decline that fails under load — Google notes AC units fail more frequently in hot months, and each return drains the thermostat's battery further until it disconnects or shuts down. Treat a returning E195 as an equipment fault with a thermostat witness, not a thermostat fault.

E294 — R wire is required

E294 is a configuration code wearing a power code's clothes, and reading it correctly saves a lot of breaker-flipping. It belongs to Google's wire-configuration group for the newer Home-app-configured thermostats (the group is E255, E257, E258, E264, E269, E294, E405, E414 and E419), and Google states the group's cause plainly: you'll get this code if the wires in your thermostat weren't configured properly in the Home app. In other words, as far as the thermostat's setup knows, there is no R wire — the one wire every heating and cooling system uses to deliver power to the thermostat — even though a wire may be physically sitting in the base. The mismatch can run in either direction: an R wire connected at the base but never entered (or entered in the wrong place) during app setup, or wiring information carried over from a setup that no longer matches what is on the wall. That is why Google's documented fix for the whole group is not a wiring repair but a configuration rebuild: factory-reset the thermostat and add it back in the Google Home app, paying extra attention when you re-enter the wiring information. E294 must also be separated from its near neighbours, because Google documents them as different reports. E298 ('No system power', on the Nest Thermostat) and E448 (the same 'No system power' alert on the Learning Thermostat 4th gen) are genuine power failures: Google's dedicated No-power article lists their causes as incorrect wiring, a system that requires a C wire with none connected, the HVAC system powered off at the breaker, fuse box or switch or a power cut, or another equipment issue preventing the system from sending power — and its diagnostic screen is Settings > Equipment > Wiring info, where a wire that isn't sending voltage shows grey. E294 says the setup lacks an R wire; E298/E448 say the system isn't sending power. A thermostat can move between them — fix a configuration so the R wire is properly declared, and a real power loss underneath it may then report honestly as E298/E448 — which is why the order of work matters: configuration first, power second, never the reverse.

Most likely cause: The wiring was not configured properly in the Google Home app — Google's stated cause for the entire E294 group, verbatim in substance: the wires in the thermostat weren't configured properly in the Home app. The R wire the thermostat requires was not entered, was entered against the wrong connector, or the setup was completed from wiring information that doesn't match the base — a photo of a previous thermostat read wrongly, or a setup finished before the wires were final.

First check: Read the code's routing before touching anything: Google sends E294 to 'Troubleshoot wires in Home app', not to 'Troubleshoot power'. Breakers and wire-pulling are not the first moves for this code — the Home app's wiring record is.

If it keeps coming back: An E294 that returns after a verified factory reset and careful re-setup means the configuration keeps diverging from the base: wires being moved at the equipment or the base without the app record being updated, a setup repeatedly completed from an old wiring photo that no longer matches the system, or a system whose actual wiring (for example a shared or jumpered R arrangement from a previous thermostat) can't be represented as entered. Stop re-resetting — each return is telling you the record and the reality disagree somewhere a photo of the control board, compared line by line with the app, will find faster than another reset will.

E74 — No power to Rh wire detected

E74 is the heating-side member of Nest's split-power family. On a thermostat with separate Rc and Rh connectors, power is reported by wire: E73 is no power to the Rc (cooling) wire, E74 is no power to the Rh (heating) wire, and E72 is the wider report that no power wires were detected at all. A thermostat with a single R connector cannot make that split, so the same loss is reported there as E195, 'No power to R wire detected' — E74 and E195 are the same event in two wiring dialects, not two different faults. The distinction that does matter is detection versus power: in E74 the thermostat can see an Rh wire in its connector, but no power is arriving on it. That points the diagnosis upstream, at whatever feeds Rh — the system supply, the heating equipment and the wire's connection — rather than at the thermostat's heating control. And like every E code, Google classifies E74 as critical: the thermostat won't let you control heating or cooling, and the message won't go away until the cause is fixed. Google singles this family out in its hot-weather guidance: if E3, E4, E23, E24, E73, E74 or E195 appears in summer on a system that had been working, the likely story is that the HVAC system has broken down and stopped powering the thermostat, and if the loss persists the thermostat's battery drains until it disconnects from the internet or shuts down entirely. An E74 in a heating season carries the mirror-image urgency for heat — either way, the code is the equipment's power failure reported at the wall, and the thermostat is almost always the messenger, not the fault.

Most likely cause: System power off at the breaker, fuse box or system switch — Google's Troubleshoot power flow starts here: make sure the power to the system is on — the switch is usually in the breaker box or fuse box, and in some homes it looks like a light switch near the heating or cooling equipment; there may be one switch, or two, one for heating and one for cooling. Google's hot-weather section adds the classic variant: breakers left off after construction or maintenance. A heating supply that is simply switched off cannot send power down the Rh wire, and on a dual-supply system the house can have cooling power present while the heating side — and with it, Rh — is dead.

First check: Check the system power first, exactly as Google orders it: breaker on in the breaker box, fuse intact in the fuse box, and the system switch on — remembering there may be two switches, one for heating and one for cooling, and Rh belongs to the heating side. If you recently had construction or maintenance done, Google names still-off breakers as the first suspect.

If it keeps coming back: An E74 that returns after the supply was proven and the wire reseated means the heating equipment is cutting its own power intermittently: a system shutting itself down as it runs (the clogged-filter, frozen-coil and drain patterns in Google's hot-weather flow), a cover switch that loses contact under vibration, a drain that backs up only under sustained cooling load, or equipment in decline that fails when it works hardest. Each return also drains the thermostat's battery further. Read a returning E74 as an equipment fault with a thermostat witness, and hand the pattern — not just the code — to the technician.

Less common and model-specific codes

Search data shows owners also meet these codes on Nest thermostats: M24, M26, N271, TD002, TD025, TD026, TD029. They are not part of the core code list Nest publishes for most models — several are series-specific, abbreviated display readings, or codes carried over from a sister platform — so we do not publish meanings for them rather than guess. Treat them by symptom: note what the machine was doing when the code appeared (filling, draining, heating, spinning), run one full power cycle (5 minutes off), and if the code returns, match the symptom to the closest documented family in the table above and check the tech sheet stored inside your machine for the model-specific definition.

Deep-dive guides for the most searched codes

Frequently asked questions

Related guides

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