E37
Google Nest Thermostat
Severity: ModerateWhat Does This Error Mean?
Nest error E37 means the thermostat is detecting an abnormally low power level coming in from your HVAC system wiring. The Nest runs on 24-volt power delivered through your thermostat wires. When that voltage drops too low, the Nest cannot operate reliably and shows E37 as a warning. This is often caused by a weak or failing HVAC transformer, a missing C-wire, or a wiring issue that is increasing resistance on the circuit. It is closely related to E36 but specifically points to a voltage problem rather than a battery problem.
Affected Models
- Nest Learning Thermostat (3rd gen)
- Nest Learning Thermostat (4th gen)
- Nest Thermostat E
Common Causes
- The HVAC system's 24-volt transformer is aging, undersized, or failing and cannot maintain proper voltage
- No C-wire is connected and the Nest is struggling to get enough power by borrowing from other wires
- A wire connection is loose or corroded at the Nest base or the furnace control board, increasing electrical resistance
- Additional devices (humidifiers, zone controllers, UV lights) sharing the transformer are overloading it
- A short circuit somewhere in the thermostat wiring is causing a voltage drop
How to Fix It
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Check your Nest's power readings. Go to Settings > Technical Info > Power. Look at the Vin value — this is the voltage coming in from your HVAC wiring. It should be 30V or higher. If it reads below 25V, the transformer or wiring is the problem.
The Nest needs a minimum of about 30V incoming to charge its battery reliably. Values below that will trigger E37 over time.
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Turn off power to the HVAC system at the breaker. Check all wiring connections at the Nest base. Pull each wire connector gently — it should hold firmly. Push any loose ones firmly back in. Look for corroded or discolored wire ends.
Corrosion on thermostat wires increases resistance in the circuit, which reduces the voltage the Nest receives. Clean corroded wire ends with fine sandpaper or replace the wire entirely if badly corroded.
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Check whether a C-wire is connected at the Nest. Go to Settings > Equipment > Wires. If no wire is detected in the C terminal and your HVAC system has a C terminal on the control board, run or connect a C-wire.
The C-wire allows proper two-wire power delivery to the Nest. Without it, the Nest uses a less efficient charging method that produces lower effective voltage and triggers E37.
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Check if other devices are sharing your HVAC transformer. Humidifiers, dehumidifiers, UV lights, and zone controllers all pull power from the same 24V transformer. If too many accessories are connected, the transformer may be overloaded.
A standard HVAC transformer is rated for 40VA. Each accessory adds load. A technician can measure transformer load and replace it with a higher-rated unit if needed.
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If you cannot run a C-wire, install the Nest Power Connector accessory inside your air handler or furnace. It creates a proper power connection for the Nest without needing to run a new wire through the wall.
The Nest Power Connector is available from the Google Store and from most HVAC supply stores. Installation instructions are included and most homeowners can install it themselves.
When to Call a Professional
Testing transformer voltage requires a multimeter and understanding of your HVAC system's low-voltage wiring. If you are not comfortable doing this, an HVAC technician can quickly measure the transformer output and check for wiring issues. Replacing an HVAC transformer is a moderately simple job but involves working near mains voltage. Technician service calls for this type of issue typically cost $75-$150 plus the cost of a replacement transformer if needed ($20-$50 for the part).
Frequently Asked Questions
How do I know if my HVAC transformer is the problem?
The most reliable way is to test the transformer output with a multimeter. With your HVAC system powered on, set your multimeter to AC voltage and touch the probes to the R and C terminals on the control board. A healthy transformer will read between 24V and 28V AC. If it reads significantly below 24V under load, the transformer is weak and should be replaced. A technician can do this test quickly during a service visit if you are not comfortable using a multimeter.
Can I replace the HVAC transformer myself?
It is possible for a confident DIYer. HVAC transformers are inexpensive ($15-$50) and widely available at hardware stores. They are the brick-shaped component on your furnace or air handler control board with two sets of wire connections. However, the primary (input) side of the transformer connects to 120V mains power. Working near mains voltage is dangerous if you are not experienced. If in doubt, have an HVAC technician replace it.
Will E37 affect my heating and cooling right away?
Not immediately — the Nest has an internal battery that provides backup power when incoming voltage is low. However, if the voltage issue is not resolved, the battery will drain over time and the thermostat will eventually shut down. You may notice the Nest screen going dim, the Wi-Fi disconnecting more often, or the thermostat going unresponsive before it fully shuts down. Address E37 before it progresses to a complete shutdown.