E12
Google Nest Thermostat
Severity: ModerateWhat Does This Error Mean?
Nest error E12 means the Nest sent a heating signal to your furnace but detected no response — the furnace did not start. The Nest monitors whether the temperature actually changes after sending a heating command. If the temperature does not rise within the expected time, it concludes the furnace is not responding. This could mean the furnace has tripped a safety limit, its power is off, or the wiring between the Nest and furnace is faulty.
Affected Models
- Nest Learning Thermostat (1st, 2nd, 3rd gen)
- Nest Thermostat E
- Google Nest Thermostat (2020)
Common Causes
- The furnace has tripped an internal safety limit — the high-limit switch, flame rollout switch, or pressure switch
- The furnace circuit breaker has tripped or a fuse in the furnace control board has blown
- The W wire (heating wire) from the Nest to the furnace is disconnected, broken, or corroded
- The furnace gas supply is off or the pilot light or igniter has failed
- The furnace drain line is blocked (on high-efficiency furnaces), which triggers a safety shutdown
How to Fix It
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Check the furnace power switch. Most furnaces have a wall switch (looks like a light switch) near the unit. Make sure it is on. Also check the furnace's circuit breaker in your main electrical panel — it may have tripped.
Furnace circuit breakers trip occasionally, especially at the start of heating season when the furnace runs hard for the first time.
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Check the furnace control board for a blown fuse. Open the furnace access panel (turn off the power first). Look for a small automotive-style fuse on the control board, usually labeled 3A or 5A. Replace it if blown.
A blown control board fuse is a common cause of E12 — it kills power to the thermostat circuit, preventing the furnace from receiving or responding to heating signals.
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Check your furnace's air filter. A severely clogged filter causes the furnace to overheat and trip the high-limit switch, which shuts the furnace down. Replace the filter if it is visibly dirty. After replacing, the high-limit switch usually resets automatically after the furnace cools — wait 30 minutes then try heating again.
High-limit switches are a safety feature that prevents furnace fires from overheating. If this trips repeatedly, the filter or blower motor is the likely cause.
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On high-efficiency condensing furnaces, check the condensate drain line. These furnaces produce water vapor that must drain away. If the drain is blocked, a pressure switch shuts the furnace down. Pour water down the drain to check for blockages, or use a wet/dry vacuum on the drain outlet.
High-efficiency furnaces have an AFUE rating of 90% or higher. They can be identified by their white PVC exhaust pipe rather than a metal flue pipe.
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Check the W wire connection at both the Nest and the furnace. This is the wire that carries the heating signal. It should be in the W or W1 terminal at both ends. A loose or corroded connection means the heating signal never reaches the furnace.
Turn off the HVAC breaker before checking wiring. Take photos before making any changes.
When to Call a Professional
E12 means the furnace itself is not working — not just the thermostat. For gas furnace safety system trips (limit switch, pressure switch, flame sensor issues), call a licensed HVAC technician. Do not attempt to bypass safety limits. If you smell gas, leave the house and call the gas company before anyone else.
Frequently Asked Questions
How does Nest know the furnace is not responding?
Nest monitors the temperature in the room after sending a heating command. If the temperature does not start rising within a set amount of time, Nest concludes the heating system is not working and displays E12. This is smarter behavior than older thermostats, which would just keep sending a signal without knowing whether the furnace was actually running. Nest's response detection means it catches furnace problems and alerts you, rather than leaving you wondering why the house is cold.
I smell gas near my furnace. What should I do?
Leave your home immediately without turning any lights or switches on or off. Do not use your phone inside the house. Once outside, call your gas company's emergency line from your mobile phone. Do not re-enter the house until the gas company has inspected it. A gas smell means a gas leak, which is a fire and explosion hazard. Never attempt to investigate a gas smell yourself.
My furnace makes a clicking sound and then shuts off. Is that related to E12?
Yes, very likely. That clicking sound is the igniter trying to light the burner and failing. After several failed ignition attempts, the furnace's control board locks out and shuts everything down — which the Nest then reports as E12. Possible causes include a faulty igniter, a dirty flame sensor, or insufficient gas pressure. A dirty flame sensor is a very common and inexpensive fix — a technician can clean it in minutes. The igniter itself costs $20-$50 to replace and is a common furnace repair.