What Does Nest Thermostat Error E73 Mean (No Power to Rc) and How Do I Fix It?

Problem Description

Your Nest thermostat shows 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. E73 shows up most in hot weather, when an overworked AC shuts itself down — a tripped safety lockout, a full condensate pan, or a frozen coil — and stops sending power to the thermostat.

Why This Happens in Real Homes

E73 means the Nest lost 24V on its Rc (cooling) wire, and in the real world that's usually the AC shutting itself down rather than a thermostat fault — which is why it spikes in hot weather. A safety lockout after failed starts, a full condensate pan tripping the float switch, or a frozen coil from a dirty filter all cut the power the Nest depends on.

Start by power-cycling the HVAC at the breaker to clear any lockout, then clear the condensate drain, change the filter, and check the outdoor disconnect. If the outdoor fan won't spin on a cool call, a failed capacitor or contactor is likely and needs a pro.

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

• No 24V reaching the Rc (cooling) wire
• AC safety lockout after failed starts
• Condensate drain pan full, tripping the float switch
• Frozen evaporator coil from a clogged filter shutting the system down
• Tripped outdoor disconnect or AC breaker
• Blown low-voltage fuse on the air-handler board
• Loose or miswired Rc/C wire after service
• Failed capacitor or contactor at the outdoor unit

Tools & Requirements

Step-by-Step Solution

1

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...

This helps complete the fix you are currently reading.

$45.99

View Needed Item

2

Check 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.

3

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. Make sure the furnace door panel is firmly seated.

Needed for this step

Levoit Vital 100 Air Purifier Genuine Replaceme...

This helps complete the fix you are currently reading.

$54.99

View Needed Item

4

Verify 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.

5

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.

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.