Nest Thermostat Error E105 Solved: Detailed DIY Repair

Quick Specs: Fix Error E105

• Difficulty: Moderate (Basic electrical handling required)
• Estimated Time: 45 – 60 Minutes
• Tools Needed:
  • Digital Multimeter
  • Phillips-head & Flat-head Screwdrivers
  • Needle-nose Pliers or Wire Strippers
  • Bright Flashlight
• Estimated Cost: $0 – $40 (depending on if a relay or wire needs replacing)

The Nest Thermostat Error E105 is a specific safety alert indicating an “overcurrent” on the G wire, which controls your fan. Essentially, your Nest has detected that the fan circuit is drawing more than 1.2 amps of electricity, and it has shut down that connection to prevent internal damage to the thermostat’s delicate circuitry.

When this happens, you might notice your home getting stuffy because the blower fan won’t kick on, or you might hear a faint clicking sound as the system tries—and fails—to start. You might even see a “Low Battery” warning because the error is preventing the Nest from charging properly. But don’t you worry, friend! While “overcurrent” sounds intimidating, it’s usually just a sign of a loose wire or a component that needs a little TLC. We can walk through this together and get your air moving again in no time!

Troubleshooting & Replacement Instructions

Alright, partner, it’s time to get to work! Follow these steps carefully, and remember: safety is our number one priority.

Step 1: Power Down for Safety
Before you touch a single wire, go to your home’s breaker panel. Find the switch labeled “Furnace,” “Air Handler,” or “HVAC” and flip it to the OFF position. Safety Warning: Working with live wires can be dangerous. Always verify the power is off by checking that the thermostat screen is dead or the HVAC lights are out.

Step 2: Inspect the Nest Base Plate
Gently pull the Nest display off the wall. Look at the wires plugged into the base. Use your flashlight to see if there are any “whiskers”—tiny copper strands—sticking out of the G terminal and touching the Y or C terminals. If you see any, use your needle-nose pliers to tuck them in or re-strip the wire so only the necessary amount of copper is exposed. A single stray hair of copper is enough to cause an E105!

Step 3: Check the Wiring at the HVAC Control Board
Head over to your furnace or air handler. You’ll need to unscrew the service panel (usually using a Phillips-head screwdriver). Locate the control board where the thermostat wires connect. Look for the “G” terminal. Check for any burnt marks, loose connections, or signs of mouse damage (chewed wires). Tighten the screw on the G terminal to ensure a solid, clean connection.

Step 4: Use Your Multimeter to Check Resistance
Set your multimeter to the Ohms (Ω) setting. Touch one probe to the G wire (disconnected from the Nest) and the other to the C (Common) wire or a metal ground. If the reading is very low (near zero), you have a short circuit somewhere in the wall. If the reading is between 20 and 100 Ohms, your relay is likely the problem. Safety Warning: Do not perform resistance tests while the power is on, as this can damage your meter.

Step 5: The “Swap Test” (Advanced)
If you’re still stuck, try swapping the G wire and the Y wire at both the thermostat and the furnace. If the error moves from E105 to an error on the Y wire, you know the physical wire in the wall is damaged. If the error stays on the fan circuit, the relay on your furnace control board is the culprit and likely needs to be replaced by a professional or a very confident DIYer.

Step 6: Power Back Up
Once you’ve tightened connections and ensured no wires are touching, snap the Nest display back on and flip the breaker back on. Give the Nest 5 minutes to reboot and run its internal diagnostics.

Detailed Diagnosis: Root Causes

Before we roll up our sleeves, let’s understand why your Nest is throwing this tantrum. There are usually three main culprits behind a G-wire overcurrent:

1. Physical Wire Shortage (The Most Common): Over time, the vibration of your HVAC system can cause wires to rub against metal frames or each other. If the insulation on your G wire (usually the green one) has worn thin and touches another wire or the metal chassis, it creates a “short.” This allows too much electricity to flow at once, triggering the E105 safety shut-off.

2. A Failing Fan Relay: Inside your furnace or air handler, there is a small electronic switch called a relay. This relay is what the Nest “talks” to. If the coil inside that relay starts to fail or gets “sticky,” it can draw significantly more power than it’s supposed to. Your Nest senses this extra “pull” and cuts the power to save itself from melting.

3. Moisture and Corrosion: If your HVAC unit is in a damp basement or near an attic leak, corrosion can build up on the control board terminals. This oxidation creates resistance, which in turn forces the system to pull more current to overcome the “gunk.” It’s essentially the electrical version of trying to breathe through a clogged straw.

4. Incompatible Equipment: Occasionally, older “high-draw” fan systems aren’t perfectly compatible with the Nest’s solid-state switching. If your system is very old, it might naturally pull close to that 1.2-amp limit, and any slight fluctuation triggers the error.

Symptoms of an E105 Overcurrent Error

Identifying the problem is the first step toward a cozy home! If your Nest is struggling with an E105 error, you will likely encounter one or more of the following signs:

• The “Frozen” Fan: The most obvious sign is that your HVAC system’s blower fan refuses to turn on, even if the AC or Heat seems to be “running” on the display.
• The Red Blink: You may see a blinking red light at the top of your Nest unit, or the screen might be completely blank because the overcurrent has tripped a safety that stops the battery from charging.
• Lukewarm Air: Since the fan isn’t moving air across the coils or heat exchanger, you might feel a tiny bit of cold or warm air drifting out of the vents, but there’s no “push” behind it.
• Error Messages: A literal “E105” or “Overcurrent on G wire” message will appear in the Nest “Equipment” settings menu.

How to Prevent Error E105

Now that we’ve got your home back to the perfect temperature, let’s make sure this doesn’t happen again! Maintenance is the key to a happy Nest.

• Change Your Air Filters Regularly: This sounds simple, but a clogged filter makes the blower motor work much harder. This extra strain can heat up the wires and the relay, leading to overcurrent issues. Change them every 30-90 days!
• Install a “Common” (C) Wire: If your system doesn’t have a C-wire, the Nest has to “power steal” from the other wires. This can put unnecessary stress on the G-wire circuit. Adding a Nest Power Connector or a dedicated C-wire provides a stable path for electricity and prevents many errors.
• Annual HVAC Checkups: Once a year, have a technician look at your control board and blower motor. They can spot a failing relay or a fraying wire long before your Nest shuts down in the middle of a heatwave.

Frequently Asked Questions

Q: Can I just ignore the E105 error if my AC is still cold?
A: I wouldn’t recommend it, friend! While the AC coils might get cold, without the fan (G wire), that cold air won’t circulate. More importantly, an overcurrent means heat is building up somewhere it shouldn’t. Ignoring it could eventually lead to a blown fuse on your furnace board or a fried thermostat.

Q: My wires look fine, but I still get the error. Is my Nest broken?
A: It’s rare, but sometimes the Nest base plate itself develops an internal fault. If you’ve checked the wires and the furnace relay and everything is perfect, try contacting Google Nest support. They can help you determine if the base plate needs a replacement under warranty.

Q: Is the G wire only for the fan?
A: Generally, yes. In most standard systems, the G wire is the signal that tells the blower motor to turn on. If you have a high-tech “variable speed” fan, the wiring might be more complex, but for 90% of homes, G equals Fan.