Nest Thermostat Error Codes: E73, E74, E195 and N Codes

By Ray Bennett · Updated 2 October 2026 · 7 min read · Checked against the maker’s manual

Nest Thermostat Error Codes: E73, E74, E195 and N Codes

Short answer

An E code stops a Nest controlling your system until the cause is fixed, and nearly every one is a wire with no power or a wire in the wrong place. E73, E74 and E195 in summer usually mean the cooling system itself stopped, E10 and its family in winter are the air conditioner's cold weather cut-out, and N codes are low power warnings.

A Nest thermostat shows a help code when it detects a problem, and the letter tells you how serious it is. Google says E codes are critical: the thermostat will not let you control heating or cooling, and the message stays until the cause is fixed. N codes are warnings, most of them about low power on a wire.

Everything on this page comes from Google’s own Nest help pages for thermostat help codes. One thing trips people up: the same fault has several numbers, one for each Nest model, so E4, E24 and E74 all mean the same thing. The tables group them.

Before you start

What to carry: a torch, your phone for a photo of the wires, and a USB cable to charge the display. Fifteen minutes.

Gas or electric? It makes no difference to the codes. The Nest runs on the low voltage power your furnace, boiler or air handler supplies, whatever fuel heats the house.

Take a photo before you touch a wire. Google says you may need the picture later, and the app’s wiring diagram depends on knowing where each wire was.

Power off before the display comes off. Turn off the system at the breaker or fuse box. Google says to do this before exposing or adjusting any wiring, and warns that a system can have more than one breaker. Turn them all off.

The furnace side carries high voltage. Google calls opening the equipment an advanced step, and says some components store a charge like a battery. Never remove a coil panel sealed with metal tape or sealant.

What the number is telling you

Almost every Nest code is one of four things: a wire with no power, a wire in the wrong place, the winter cut-out on Y1, or a low power warning.

Check 1Free, two minutes

Find your code's family

No power on a wire. The thermostat cannot find power on Rc, Rh, R, C, W1 or Y1. Google’s first step is to check the system’s power switch or breaker, which in some homes looks like a light switch near the equipment.

A wire missing or in the wrong place. Such as a heat pump without Y1, or Y2 without Y1. The fix is to match the wires to the wiring diagram the app gives you: where each wire goes on a Nest.

Winter, no power on Y1. Google says an air conditioner can carry a safety cut-out that stops it in very cold weather, and the Nest loses the power it borrows through Y1.

Low power warnings. The N codes. Google says that if the system is running normally you can dismiss the message and keep using the thermostat.

Every E code, grouped

Google lists these, with the section of its help to follow for each.

Codes	What Google says it means	Where to look
E1	A common wire (C wire) is required	C wire
E2, E22, E72, E402	No power wires detected, Rc or Rh required	Power
E3, E23, E73	No power to the Rc wire	Power
E4, E24, E74	No power to the Rh wire	Power
E5, E25, E75, E155, E255, E405	No heating or cooling detected, W1 or Y1 required	Wiring
E6	Too many wires, remove one of O/B, W1, Y1 or AUX	Wiring
E7, E27, E77, E407	Y1 not detected, a heat pump requires Y1	Wiring
E8, E28, E78, E408	W2/AUX detected by itself	Wiring
E9, E29, E79, E159	No power to the C wire	Power
E10, E30, E66, E80, E116, E160, E196, E296	No power to Y1, the cold weather shutoff	Y wire
E11	O/B wire detected with no heat pump	Wiring
E12, E32, E82, E162	No power to the W1 wire	Power
E13, E33, E36, E37, E38, E83, E86, E87, E88, E413	Too many auxiliary heat wires for a heat pump	Wiring
E14, E34, E84, E164, E264	Dual fuel requires an alternate heat wire	Wiring
E39, E89, E169, E269, E419	Y2 wire without a Y1 wire	Wiring
E52 to E58, E102 to E108, E182, E183, E188	Overcurrent on your heating or cooling equipment	Wiring, then a pro
E194	R wire not detected	Wiring
E195	No power to the R wire	Power
E297	A C wire or Y1 wire is required	C wire
E298, E448	No system power	Power
E299	Contact support	Google support

E73, E74 or E195 in summer

In hot weather these codes usually mean the cooling system stopped powering the thermostat. Google's checks run from the breakers to a professional.

Check 2

Work Google's hot weather checks

Why summer. Google says the system can break down in hot months and stop powering the thermostat. If it goes on, the Nest’s battery drains and it may go offline or shut down.

The breakers. Check every breaker is on, especially after building or maintenance work in the house.

The wires at the Nest. Power off, then take each wire out in turn. Google wants it straight, clean, not corroded, with copper showing, and the connector button staying down when it goes back.

The air filter. A clogged filter starves the cooling coil of air until it freezes solid and the system shuts off. Change it, and give a frozen coil time to thaw: why an air conditioner freezes up.

The drain pan and its switch. Google says a clogged drain can back water up, and a float switch then shuts the system off, taking the thermostat’s power with it: how to clear a clogged drain line.

The fuse on the furnace board. Google describes a small U-shaped fuse marked 3, often pink or purple, and says to replace it if it is blackened, discoloured or broken inside.

E10, E30 and the winter codes

Check 3Free, fifteen minutes

Get the heat back, then fix it properly

What is happening. Google says a Nest can power itself through the Y1 wire, which runs to the air conditioner. A cold weather safety device, sometimes a low pressure switch, can stop the air conditioner and with it that power.

Google’s quick fix. Power off, take the display off, photograph the wires, then disconnect the Y wires and tape each bare end so it cannot touch anything. Google’s page gives the exact steps for your setup.

If the screen stays dark. Charge the display by USB for 5 to 10 minutes, or fit new AAA batteries on models that take them.

What you give up. Google says the air conditioning will not work until Y1 is reconnected, so this is a winter measure.

The real fix. A C wire or the Nest Power Connector, so the thermostat stops borrowing power: why a Nest without a C wire misbehaves.

The N codes

Google lists these as low power and advice codes. Most can be dismissed if the system is working.

Codes	What Google says it means
N1, N21, N71, N401	Rc detected but not connected to equipment
N2, N22, N72, N402	Rh detected but not connected to equipment
N3, N23, N73, N153, N403	No power to the W1 wire
N4, N24, N74, N154, N404	No power to the Y1 wire
N5, N25, N75, N405	No power to the W2/AUX wire
N6, N26, N76, N156, N406	No power to the G wire
N7, N27, N77, N407	No power to the O/B wire
N28, N78, N408	No power to the Y2 wire
N260, N410	A C wire or Nest Power Connector is recommended
N261, N411	Low power to the C wire
N262, N263, N412, N413	In extreme cold only, no power to Y1; check the batteries
N416 to N423	Overvoltage or overcurrent on your equipment

When it needs a professional

Check 4

Know when to stop

The overcurrent codes. E52 to E58, E102 to E108, E182, E183 and E188 mean the Nest saw too much current on your equipment, and Google sends you to the wiring checks first. For N416 to N423, Google says the system may need maintenance.

A summer power code with nothing found. Google says that if the breakers, filter, drain, fuse and wiring all look right, a professional should find what stopped the system.

No spare wire for a C wire. Google says that if there is no unused wire in the cable, a pro needs to run one, or you can fit the Nest Power Connector.

E299. Google’s only instruction is to contact its support team.

Questions people ask

What does E73 mean on a Nest thermostat?

No power to the Rc wire. In summer, Google says it usually means the cooling system broke down and stopped powering the thermostat. Check the breakers, the filter, the drain and the fuse, then call a professional.

What does E74 mean?

No power to the Rh wire. It belongs to the same family as E4 and E24, and the same checks apply: breakers first, then the wiring at the thermostat.

What is E195 on a Nest?

No power to the R wire. Google groups it with E73 and E74 as a hot weather power error, so start with the breakers and the cooling system.

What does E298 mean?

No system power. Check the breaker or power switch for the heating and cooling equipment, then the wires at the thermostat.

Can I ignore an N code?

Google says that if your system is running normally, you can dismiss the message and keep using the thermostat. It should not come back after you dismiss it.

Why are there so many numbers for one fault?

Google’s table gives each Nest model its own column, so one fault, such as no power on Rh, appears as E4, E24 and E74. The meaning is what matters.

Sources: Google Nest Help: "Troubleshoot Nest thermostat help codes" and "Troubleshoot E10, E30, E66, E80, E116, E160, E196, E296, M13, N262, N263, N412, and N413 help codes"