E39
Google Nest Thermostat
Severity: ModerateWhat Does This Error Mean?
Nest error E39 means your thermostat is not receiving enough power from the HVAC system's wiring. The thermostat's battery is too low to operate correctly and the system is not providing enough current to keep it charged. This usually points to a missing common wire (C-wire) or a problem with your HVAC system's control board.
Affected Models
- Nest Learning Thermostat (3rd Gen)
- Nest Thermostat E
- Nest Thermostat (2020)
- Google Nest Thermostat
Common Causes
- No C-wire (common wire) is connected, leaving the thermostat with no continuous power source
- The C-wire is connected but has a loose or corroded terminal connection
- The HVAC system's control board is not supplying the correct 24V AC power
- A blown fuse on the HVAC control board has cut power to the thermostat circuit
- Power is being stolen from another wire causing irregular voltage that cannot charge the thermostat
How to Fix It
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Check the Nest app or the thermostat display for a low battery warning alongside E39. If the battery is low, it may just need time to recharge.
Leave the thermostat in place for 30 minutes with the HVAC system on. If it has a C-wire, the battery should begin to recharge on its own.
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Remove the Nest thermostat from its base and check the battery level. If critically low, charge it using a USB cable (Micro-USB for most models).
The charging port is on the back of the thermostat display. A full charge takes about 30 minutes via USB.
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Check whether a C-wire (common wire) is connected at the thermostat base. Look for a wire in the terminal labeled C.
Without a C-wire, Nest thermostats cannot receive continuous power and must steal power from other wires — a process that often causes this error.
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If you have a C-wire, pull it out slightly and re-insert it firmly into the C terminal to ensure solid contact.
Loose terminal connections cause intermittent power supply. Push the wire in until you feel it lock.
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If there is no C-wire available, install a Nest Power Connector (also called a common wire adapter). This device is included with some Nest models or available separately.
The Nest Power Connector installs at your HVAC system's air handler or furnace and provides a virtual C-wire without new wiring. Instructions are included in the box.
When to Call a Professional
If you have verified the C-wire is connected properly and the error persists, the issue may be with your HVAC control board — specifically the 24V transformer or a blown fuse. An HVAC technician can test the board's output voltage and replace a blown fuse or failed transformer. Do not attempt to modify HVAC wiring if you are not familiar with low-voltage thermostat wiring.
Frequently Asked Questions
What is a C-wire and why does my Nest need it?
The C-wire (common wire) provides a continuous 24V power supply to the thermostat. Without it, the Nest must steal tiny amounts of power from other wires, which is unreliable and causes charging errors like E39. Not all homes have a C-wire — if yours does not, the Nest Power Connector accessory solves this without rewiring.
My Nest shows E39 but I have a C-wire — why?
The C-wire may be loose at either the thermostat base or the HVAC system end. Check both connection points and re-insert the wire firmly. Also check the HVAC control board for a blown fuse — a blown fuse cuts the 24V supply entirely. Fuses are typically found near the wiring terminals on the control board and cost about $5 to replace.
Will error E39 cause my heating or cooling to stop working?
It can — if the thermostat battery drains completely, the thermostat will shut down and stop controlling your HVAC system. You may notice your home no longer heating or cooling even though the system appears on. Charge the thermostat via USB and resolve the power issue to restore normal operation.