E5
Google Nest Thermostat
Severity: CriticalWhat Does This Error Mean?
Nest error E5 means the thermostat sent a command to start the heating system, but the system did not respond as expected. The Nest monitors the heating equipment's response after sending a heat request. If the system does not start, runs for too short a time, or fails to produce heat, E5 appears. This can point to a wiring problem, a tripped safety switch inside the furnace, or a furnace fault.
Affected Models
- Nest Learning Thermostat (1st, 2nd, 3rd gen)
- Nest Thermostat E
- Google Nest Thermostat (2020)
Common Causes
- The W (heat) wire is disconnected, loose, or not making proper contact at the Nest or at the HVAC control board
- The furnace has a separate fault and has shut itself down — its own error lights or display may show the cause
- A blown fuse on the HVAC control board is preventing the heating command from reaching the furnace
- The furnace pilot light is out (on older gas furnaces with standing pilots) or the igniter has failed on newer systems
- The furnace has a safety lockout due to too many failed ignition attempts
How to Fix It
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Check the furnace or air handler directly. Go to the unit and look for any indicator lights or error codes on the unit's own control board display. Most modern furnaces have a small window where a flashing LED shows error codes — these give you direct information about the furnace fault.
The furnace's own error codes tell you exactly what is wrong at the equipment level. Look up your furnace brand and the flashing code pattern (e.g., 3 flashes = specific fault) in the furnace manual or online.
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Turn off HVAC power at the breaker and check the W wire at the Nest base. Pull the Nest off the base and confirm the W or W1 wire is fully seated in its terminal. Also go to the HVAC unit and check that the wire is properly connected at the control board.
A loose W wire at either end will prevent the heating command from completing the circuit. The Nest sends the heat command over this wire — if the circuit is broken, the furnace receives nothing.
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Check the HVAC control board fuse. Turn off power at the breaker first. On the HVAC control board (typically inside the air handler or furnace cabinet), look for a small automotive-style fuse — usually 3A or 5A. If it is blown (the metal strip inside is broken), replace it with the same rating.
A blown control board fuse is a common and inexpensive fix. Replacement fuses cost under $5 at hardware stores. Always use the same amperage rating — never use a higher-amp fuse as a substitute.
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Reset the furnace. Some furnaces have a dedicated reset button — usually a red button on the burner housing or near the motor. Hold it in for 5 to 10 seconds. Then restore HVAC power and set the Nest to call for heat. The furnace will attempt to restart.
Furnaces that have had too many failed ignition attempts enter a lockout mode and stop trying. A reset clears the lockout and gives the furnace another chance to start.
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Call an HVAC technician if the furnace does not start after a reset or if the furnace's own error codes indicate a component failure. Describe both the Nest E5 error and any furnace error code flashes you observed.
Never attempt to repair gas furnace components — gas line work, igniter replacement, and gas valve service require licensed professionals for safety reasons.
When to Call a Professional
E5 on a gas or oil furnace almost always requires a technician. Furnace igniter failures, gas valve problems, and control board faults are not DIY repairs. Call an HVAC technician, especially in cold weather when heating failure is an emergency.
Frequently Asked Questions
My furnace error light is flashing. What does that mean?
Most modern furnaces use a small LED on the control board to show error codes. The number of flashes (or a combination of long and short flashes) corresponds to a specific fault. The decoding chart is usually printed on a sticker inside the furnace access panel. Common codes include: 3 flashes = pressure switch fault, 4 flashes = open high-limit, 7 flashes = flame sense failure. Look up your furnace brand and model with the specific flash pattern for an exact diagnosis.
Can I keep using the Nest for cooling while E5 is active?
Yes — E5 is specific to the heating system. If your cooling system (air conditioner or heat pump in cooling mode) is wired separately and functioning, you can still use the Nest for cooling. However, during cold weather a heating failure is urgent. Address the heating issue as soon as possible, especially if temperatures are near or below freezing.
How do I know if my igniter is bad?
On a gas furnace with electronic ignition, a bad igniter will cause the furnace to attempt startup — you may hear the inducer fan come on — but then fail to produce a flame. The furnace will try 2 to 3 times and then lock out. You can visually inspect the igniter (it looks like a small silicon carbide element or a hot surface element near the burners) by watching through the burner inspection port, but replacement should be done by a technician.