E3
Google Nest Thermostat
Severity: ModerateWhat Does This Error Mean?
Nest error E3 means the HVAC fan is running continuously when the Nest has not commanded it to run. In other words, the Nest told the fan to turn off, but it is still blowing. This wastes electricity, puts wear on the fan motor, and means the HVAC system is not under proper thermostat control. It is usually a wiring issue or a problem with the fan relay on the HVAC control board.
Affected Models
- Nest Learning Thermostat (1st, 2nd, 3rd gen)
- Nest Thermostat E
- Google Nest Thermostat (2020)
Common Causes
- The G wire (fan control wire) has a short or is connected to a terminal that keeps the fan circuit energized
- The fan relay on the HVAC control board is stuck in the closed (on) position
- A wiring error is connecting the fan circuit to a constantly powered terminal, bypassing thermostat control
- The HVAC system has a manual fan-on switch (often on the air handler) set to ON instead of AUTO
- The Nest's own fan setting is set to run the fan continuously — this is a settings issue rather than a fault
How to Fix It
-
Check the Nest fan setting. On the Nest, press the ring to open the menu and navigate to Settings > Fan. Make sure the fan is not set to run continuously ('Always On'). Set it to 'Auto' or to a minimal schedule if you prefer some air circulation.
This is the simplest and most common cause of E3 — the fan is running as instructed, but it was set to 'Always On' by accident. Changing to 'Auto' immediately stops the continuous fan operation.
-
Check the fan switch on the HVAC air handler or furnace. There is usually a manual switch on the unit itself labeled FAN with positions for AUTO and ON. If it is set to ON, the fan runs regardless of thermostat commands. Set it to AUTO.
This switch is commonly found on the air handler's cabinet, near the blower section. Some homeowners accidentally switch it to ON during maintenance and forget to change it back.
-
Turn off HVAC power at the breaker and inspect the G wire (fan) at the Nest base. Pull the Nest off the base and check that the G wire is only connected to the G terminal and is not bridged to the R (power) terminal. A G wire touching the R terminal creates a constant fan-on circuit.
Even slight contact between wire strands from different terminals can create the short that keeps the fan running. Trim back any stray wire strands and ensure each wire is fully seated only in its own terminal.
-
Temporarily disconnect the G wire from the Nest base (with power off) and restore power. If the fan stops running without the G wire connected, the G wire circuit has a fault between the thermostat and the HVAC unit. This points to a wiring short or a stuck fan relay.
This diagnostic test tells you whether the problem is in the wiring/relay or in the thermostat itself. If the fan stops without the G wire, the fault is in the wiring or control board — not the Nest.
-
Call an HVAC technician if the fan relay is suspected. With power off, the technician can measure resistance across the fan relay to confirm if it is stuck closed. A stuck relay is an inexpensive part but requires access to the HVAC control board to replace.
Fan relays are small components that can stick in the closed (ON) position after years of use, especially if the relay was ever subjected to a power surge.
When to Call a Professional
If wiring checks and settings look correct but the fan runs continuously, the HVAC control board's fan relay has likely failed. This requires an HVAC technician to test and replace the relay or control board. Expect $150 to $400 for a relay or board repair.
Frequently Asked Questions
Is it harmful to run the fan continuously?
Running the fan continuously is not dangerous, but it is inefficient. It increases your electricity bill since the fan motor runs 24/7 instead of only when needed. It also circulates air more than necessary, which can dry out your home and increase filter clogging. Some homeowners choose to run the fan continuously for better air circulation — this is fine if that is intentional, but E3 indicates the thermostat has lost control of the fan, which is a problem.
Could E3 be related to my air conditioning?'
Yes — air conditioner systems typically run the fan while cooling is active and stop the fan shortly after the compressor shuts off. If E3 appears and the AC has been running recently, the fan may have gotten stuck on due to a relay issue triggered during a cooling cycle. Check whether E3 appears in cooling mode specifically or all the time.
My old thermostat also could not turn the fan off. Is this a Nest problem?
If the fan ran continuously with your old thermostat too, the issue is with the HVAC system — not the Nest. A fan relay that is stuck closed will keep the fan running regardless of what thermostat is installed. The Nest is simply more sophisticated at detecting and reporting this fault. Have an HVAC technician inspect the fan relay and control board.