What Does Nest Thermostat Error E73 Mean and How Do I Fix HVAC Not Responding?
- HVAC system safety switch tripped such as high limit switch or pressure switch
- Condensate drain pan full triggering the overflow safety float switch
- HVAC system locked out after too many failed ignition attempts
Problem Description
Your Nest thermostat is showing error code E73 which means the thermostat sent a command to your HVAC system to turn on heating or cooling but detected that the system did not respond. The thermostat is calling for heat or cool but the furnace, air conditioner, or heat pump is not activating. E73 indicates a communication failure between the thermostat and your HVAC equipment. Your home will not heat or cool while this error persists. The thermostat itself is functioning correctly and has power but the HVAC system is ignoring the signal.
Symptoms
- Nest thermostat displays E73 error code
- Thermostat shows heating or cooling active but no air comes from vents
- Fan runs but no heated or cooled air is being produced
- HVAC system does not turn on at all when thermostat calls for it
- Error appeared after a power outage or sudden temperature change
- System worked yesterday but now shows E73 with no heat or cool
Recognize these? Here's what usually causes it.
Common Causes
- HVAC system safety switch tripped such as high limit switch or pressure switch
- Condensate drain pan full triggering the overflow safety float switch
- HVAC system locked out after too many failed ignition attempts
- Thermostat wired incorrectly causing signals to go to wrong terminals
- HVAC circuit breaker tripped cutting power to furnace or AC unit
- Outdoor AC unit disconnect switch turned off or tripped
Most fixes happen in the first 3 steps.
If E73 keeps returning after power cycling the HVAC system there is likely a mechanical failure that requires a licensed HVAC technician. Repeated ignition failures, gas valve issues, or compressor failures cannot be fixed with thermostat troubleshooting and attempting to override safety switches is dangerous.
Tools & Requirements
These tools will help you complete this fix.
Replacement air filter
Levoit Vital 100 Air Purifier Genuine Replacement Fi...
Screwdriver for furnace panel access
STREBITO 155 in 1 Electric Screwdriver Set, Small El...
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Step-by-Step Solution
Power Cycle the HVAC System
Go to your electrical panel and find the breaker labeled Furnace, Air Handler, or HVAC. Turn it off. If you have a separate breaker for the outdoor AC unit or heat pump turn that off too. Wait 60 seconds. Turn both breakers back on. This resets any safety lockouts on the HVAC control board. Many HVAC systems lock out after 3 failed ignition attempts and require a power cycle to reset. After resetting check if the Nest thermostat clears the E73 error and calls for heat or cool successfully.
Needed for this step
STREBITO 155 in 1 Electric Screwdriver Set, Sma...
$45.99Check the Condensate Drain
Air conditioning systems and high-efficiency furnaces produce condensation that drains through a PVC pipe. If this drain clogs water backs up into the drain pan. A float switch in the pan detects the water and shuts off the HVAC system to prevent water damage. Find the drain pan under your air handler or furnace. If it has water in it the drain is clogged. Clear the clog by using a wet-dry vacuum on the drain pipe end outside or pouring a cup of distilled vinegar through the drain access point.
Check HVAC Safety Switches
Your furnace has several safety switches that shut it down if something is wrong. The high limit switch trips if the furnace overheats usually from a clogged filter restricting airflow. Check and replace your air filter if dirty. The pressure switch trips if the exhaust vent is blocked. Check the exhaust pipe outside for obstructions like ice, debris, or bird nests. The door switch shuts the system off if the furnace panel is not properly closed. Ensure the furnace door panel is firmly seated.
Needed for this step
Levoit Vital 100 Air Purifier Genuine Replaceme...
$54.99Verify Thermostat Wiring
If E73 appeared after installing the Nest or after someone worked on the wiring the thermostat may be wired incorrectly. Remove the Nest from the wall base and take a photo of the wire connections. Compare to your HVAC control board terminal labels. The wire colors and terminal letters should match at both ends. Common mistakes include swapping W1 and W2 wires or connecting the C wire to the wrong terminal. Correct any mismatches.
Check Outdoor Unit
For air conditioning and heat pump systems check the outdoor unit. Verify the disconnect switch near the unit is in the ON position. Check that the circuit breaker for the outdoor unit has not tripped. Look for ice buildup on the unit which indicates a refrigerant issue requiring an HVAC technician. If the outdoor unit fan is not spinning when the thermostat calls for cool the capacitor or contactor may have failed which requires professional repair.
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.
After resolving E73 check your air filter immediately. A clogged air filter is the single most common cause of HVAC safety shutdowns that trigger E73. Replace the filter and set a reminder to check it monthly. The Nest app can send filter reminders based on system runtime hours. **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.
- HVAC system safety switch tripped such as high limit
- Condensate drain pan full triggering the overflow safety float
- HVAC system locked out
- Thermostat wired incorrectly causing signals to go to wrong
- HVAC circuit breaker tripped cutting power to furnace or
Before you go — try one of these (they fix most cases).
Most popular upgrades chosen by Google Nest Thermostat owners.
Google Nest Thermostat Trim Kit - Made for the Nest Therm...
Google Nest Thermostat Trim Kit - Made for the Nest Therm...
Google Nest Thermostat - Smart Thermostat for Home - Prog...
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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.
