Nest Thermostat Error E104: What Is This Critical Error and How Do I Fix It?
- Repeated current spikes on the G (fan) wire tripping the Nest's internal breaker
- Two wires forced into the G connector or a strand bridging to Y or W
- Shorted or sticking blower/fan relay on the control board
Problem Description
Nest error E104 is a critical overcurrent error indicating too much electrical current is flowing through the thermostat wiring. This is a safety hazard that can damage the thermostat or HVAC equipment.
Why This Happens in Real Homes
E104 works exactly like E103 but on the G (fan) wire — the Nest detected repeated current spikes on the fan line and tripped its internal breaker. The usual culprits are a wiring fault at the thermostat (two wires in the G connector or a strand bridging to Y or W), an R-to-G jumper left over from an old thermostat, or a sticking blower relay on the control board driving the spikes.
Start with a restart to clear a one-off, then cut power and confirm the green fan wire is clean and alone in the G terminal with no stray strands. If E104 keeps coming back with the wiring correct, the fan relay or transformer is likely failing — bring in a licensed HVAC technician rather than resetting past repeated trips.
Symptoms
- Nest displays E104 error
- Critical overcurrent message
- Thermostat shut down
- Fire hazard warning
- System unresponsive
- E104 after electrical work
Recognize these? Here's what usually causes it.
Common Causes
- Repeated current spikes on the G (fan) wire tripping the Nest's internal breaker
- Two wires forced into the G connector or a strand bridging to Y or W
- Shorted or sticking blower/fan relay on the control board
- G wire pinched or rubbed through between the air handler and the wall
- An R-to-G jumper left in place from a previous thermostat
- Failing blower motor relay drawing excess current
- Undersized or failing transformer under fan load
- A one-time electrical surge that tripped the protection once
Most fixes happen in the first 3 steps.
If E104 appeared after DIY electrical work, double-check all connections. A single wire in the wrong place can cause overcurrent.
Tools & Requirements
These tools will help you complete this fix.
Multimeter
Klein Tools 80196 Digital Multimeter Kit with Case, ...
Screwdriver
STREBITO 155 in 1 Electric Screwdriver Set, Small El...
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Step-by-Step Solution
Understand what E104 is telling you
E104 works exactly like E103 but on the G wire — the fan wire. The Nest detected the current on the G line spike repeatedly and tripped its internal breaker after about 10 spikes. It is an overcurrent on the fan circuit specifically, so the fix targets the G/fan path rather than general wiring.
Restart the thermostat first
A one-time spike often trips the protection once and then clears. Restart the Nest by holding the display/ring for about 10 seconds, or via Settings > Reset > Restart in the app. If E104 does not return, it was a single event and needs nothing further.
Cut power and inspect the G wire at the thermostat
If it comes back, switch the HVAC off at the breaker, pull the Nest off its base, and check the G (green, fan) wire: about 1cm of clean straight copper, fully seated in the G connector with the button pressed, and the ONLY wire in that terminal. A second wire crammed into G, or a stray strand bridging to the Y or W terminal, is the usual cause.
Check the fan wire and blower relay at the air handler
Trace the green wire to the furnace/air-handler control board. A sticking or shorted blower/fan relay drives the current spikes the Nest is seeing, and a pinched or rubbed-through G wire between the unit and the wall does the same. Look for burnt or pinched sections and confirm the wire is firmly landed on the G terminal at the board.
Verify wiring, then call a pro if it persists
In the Nest app open Settings > Equipment and confirm the wires match the base — an R-to-G jumper left from an old thermostat is a common overcurrent trigger on the fan circuit. If the G wiring is clean and correct but E104 keeps returning, a failing fan relay, blower motor, or transformer is likely, and a licensed HVAC technician should diagnose it rather than resetting past repeated trips.
Quick Solutions
Still having issues? This is usually the deeper cause below.
Schedules that skip randomly are usually a daylight-saving holdover — delete and recreate the schedule to clear the corrupted entry.
E104 is one of the most serious Nest errors. It protects against electrical damage and fire. Never try to bypass or ignore it.
Thermostat issues that keep returning are often caused by stale backup-battery memory holding old settings across power cycles without the user realising.
- Repeated current spikes on the G (fan) wire tripping
- Two wires forced into the G connector or a
- Shorted or sticking blower/fan relay on the control board
- G wire pinched or rubbed through between the air
- An R-to-G jumper left in place from a previous
Before you go — try one of these (they fix most cases).
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Official Manufacturer Manual
Google Nest provides official product documentation through their online manual rather than downloadable PDF. Access setup guides, troubleshooting steps, and product specifications for your Google Nest Thermostat.
Source: google.com
Need More Help? Google Nest Support
Note: The contact information below connects you directly to Google Nest's official customer support team, not Trunetto. They can help with warranty claims, device replacements, and advanced technical issues.
How Does Google Nest Compare?
Before replacing your Google Nest device, see how it stacks up against alternatives in our full comparison guides.
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Guide Improvements
- Updated July 4, 2026
Corrected E104 to its specific official meaning — an overcurrent on the G (fan) wire — replacing the previous generic short-circuit content.
What changed:- Identified E104 as a G (fan) wire overcurrent per Google's official code list
- Explained the internal-breaker trip mechanism and the R-to-G jumper cause
- Rebuilt causes and solutions around the green fan wire and the blower relay
- Added restart-first and clean-single-wire-in-G steps
Source: Editorial Accuracy Review
