How Do I Fix the Nest Thermostat E73 Error Code?
- No 24V reaching the Rc (cooling) wire
- AC shut down on a frozen coil from a clogged filter in hot weather
- Condensate drain pan full, tripping the float safety switch
Problem Description
Your Nest thermostat displays error E73, which Google defines as "no power to the Rc wire" — the 24V feed for your cooling side. The thermostat cannot run cooling and may lose power itself. E73 shows up most in hot weather, when an overworked AC shuts itself down (a tripped safety, a frozen coil, or a full condensate pan) and stops sending power to the thermostat, rather than because the thermostat has failed.
Why This Happens in Real Homes
E73 is Google's code for "no power to the Rc wire" — the cooling side's 24V feed — so it clusters in hot weather when the AC is working hardest. The usual real-world story isn't a broken thermostat: an overworked AC trips off (a clogged filter freezing the coil, or a full condensate pan tripping the float switch) and stops feeding the Nest, so its battery drains and E73 appears.
Start by ruling out an AC shutdown — change a dirty filter, let a frozen coil thaw, and clear the drain — then check the outdoor disconnect, the control-board fuse, and the Rc wire. If 24V is present at the board but E73 stays, the wire or transformer needs a pro.
Symptoms
- Error E73 shown on the thermostat
- Cooling will not turn on
- Thermostat loses power or its battery drains
- Error appears during hot weather
- AC does not respond to a cooling call
- Heating may still work while cooling does not
- Thermostat goes offline or restarts
- No 24V at the Rc terminal
Recognize these? Here's what usually causes it.
Common Causes
- No 24V reaching the Rc (cooling) wire
- AC shut down on a frozen coil from a clogged filter in hot weather
- Condensate drain pan full, tripping the float safety switch
- Blown low-voltage fuse on the air-handler control board
- Tripped breaker or disconnect at the outdoor AC unit
- Loose or corroded Rc wire at the base or control board
- Missing Rc-Rh jumper on a single-transformer system
- Failing cooling-side transformer
Most fixes happen in the first 3 steps.
Always turn off your HVAC system at the breaker before removing the thermostat or touching wires. Incorrect wiring can damage both the thermostat and your HVAC equipment resulting in expensive repairs. If unsure about wiring consult an HVAC technician.
Tools & Requirements
These tools will help you complete this fix.
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Step-by-Step Solution
Understand the E73 error
Google defines E73 as no power to the Rc wire, the 24V feed for your cooling side. Without it the Nest cannot run the AC and its battery starts to drain. E73 is grouped with hot-weather power loss: when an overworked AC shuts itself down it stops sending power to the thermostat, so E73 is usually a symptom of an AC problem rather than a thermostat fault.
Check the cooling-side power
Confirm both the indoor air-handler breaker and the outdoor condenser disconnect switch and breaker are on. A flipped outdoor disconnect or tripped breaker cuts Rc power and triggers E73. Look for a status LED on the control board; if it is dark, no power is reaching it.
Rule out a hot-weather shutdown
The top summer trigger is the AC protecting itself. A clogged filter freezes the evaporator coil and the system stops, so replace the filter and let any ice thaw for a few hours. Then check the condensate pan: a clogged drain trips a float switch that cuts power on purpose. Clear the drain line (a wet/dry vac on the outdoor outlet usually pulls the clog) and empty the pan.
Check the fuse and Rc wiring
Inspect the 3-5A blade fuse on the air-handler control board and replace if blown. With power off, pull the Nest and confirm the Rc wire has about 1cm of clean copper, sits fully in the Rc terminal, and lands firmly on the board. On single-transformer systems make sure the Rc-Rh jumper is in place.
Measure voltage or call a pro
With power on, meter across Rc and C at the board: 24-28V AC is normal. No voltage points to a tripped safety or a failing cooling transformer upstream; correct voltage with E73 still showing points to a broken Rc wire in the wall. Persistent E73 after these checks needs an HVAC technician.
Quick Solutions
Still having issues? This is usually the deeper cause below.
If the keypad rejects valid codes, a lockout timer may be running — five failed entries locks most keypads silently for 5–10 minutes.
Use the thermostat energy reports to find patterns in your heating and cooling usage. Setting back the temperature just 3 degrees when you leave for work can save 5 to 10 percent on your annual energy bill without any comfort sacrifice. **Product Intelligence:** - C-wire required for most models - 2.4GHz WiFi only - Nest Aware subscription for history
Thermostat issues that keep returning are often caused by stale backup-battery memory holding old settings across power cycles without the user realising.
- No 24V reaching the Rc (cooling) wire
- AC shut down on a frozen coil from a
- Condensate drain pan full, tripping the float safety switch
- Blown low-voltage fuse on the air-handler control board
- Tripped breaker or disconnect at the outdoor AC unit
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 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 E73 to its official Google meaning — no power to the Rc (cooling) wire — from the previous incorrect Rh/heating description, and reframed the guide around hot-weather AC shutdowns.
What changed:- Corrected the wire from Rh (heating) to Rc (cooling) per Google's official help codes
- Reframed causes around hot-weather AC shutdowns (frozen coil from a clogged filter, full condensate pan, tripped safety) that cut Rc power
- Rebuilt the symptoms, causes, solutions, and steps to match the Rc/cooling diagnosis
- Added outdoor-disconnect and condensate float-switch checks
Source: Editorial Accuracy Review
