Google Nest thermostat error E74 (No power to Rh) or E73 (No power to Rc) means your thermostat isn't receiving 24V AC power from your HVAC system. This usually happens when the condensate drain overflow switch trips or the furnace control board fuse blows.
Quick answer
- Clear the condensate drain line to reset the float switch that's cutting power to prevent flooding
- Replace the blown 3A fuse on the furnace control board if the metal loop inside is broken
- Install a C-wire adapter if you only have 4-wire thermostat wiring and the error happens during long AC runs
- Affects Nest Learning Thermostat (Gen 3), Nest Thermostat E, and Nest Thermostat (2020)
Symptoms
- Nest display shows E74 (No power to Rh) or E73 (No power to Rc)
- Thermostat battery drains rapidly and disconnects from Wi-Fi
- AC or heat system appears dead with no fan spinning
- Water visible in the AC drain pan under your indoor unit
Quick checks
Check for standing water in your AC drain pan (usually in attic or utility closet). If you see water, the overflow safety switch has cut power to prevent flooding.
Look at your thermostat wiring at the wall plate. Count the wires — if you only have 4 wires with no C (common) wire, power issues are common during heavy AC use.
Check the furnace fuse by opening your furnace panel and looking for a small automotive-style blade fuse (usually purple 3A) on the control board.
Step-by-step fix
Step 1: Clear the condensate drain (most common summer issue)
- Turn off power to your HVAC system at the breaker
- Locate your indoor AC unit (usually in attic, basement, or utility closet)
- Find the drain pan underneath — look for standing water
- Locate the float switch on the PVC drain pipe (small device with wires)
- Shop-vac the drain line from outside your house to remove algae and slime blockage
- Verify water drains freely from the pan
- Restore power — the float switch automatically resets when water level drops
Step 2: Replace the furnace fuse
- Turn off HVAC power at the breaker
- Open the furnace access panel (remove screws or lift cover)
- Find the control board with electronic components
- Locate the blade fuse (purple 3A or orange 5A automotive-style)
- Pull out the fuse and inspect the metal loop inside
- Replace with identical amp rating if the metal is broken
- Close panel and restore power
Step 3: Install C-wire adapter (if no common wire)
- Turn off HVAC power at the breaker
- Install Nest Power Connector at the furnace control board following included instructions
- Connect the adapter between the furnace board and existing thermostat wires
- Restore power and test thermostat operation
If it still isn't working
Test voltage at the furnace board — you should measure 24V AC between R and C terminals with a multimeter. If voltage is 0V with good fuse and dry switch, your transformer or control board has failed.
Check for broken wires if you have 24V at the furnace but 0V at the thermostat wall plate. The wire bundle may be damaged inside the wall.
Consider a short circuit if the new fuse blows immediately after installation. This means thermostat wires are touching somewhere in the system.
FAQ
Why does E74 only happen when my AC runs for hours? Your system lacks a C-wire, so the Nest tries to "power steal" from other wires. During heavy AC use, this power stealing fails and the thermostat loses power.
Can I use a higher amp fuse if 3A keeps blowing? No. Using a 5A fuse when the board calls for 3A can damage your control board. Fix the underlying short circuit instead.
What's the difference between E73 and E74? E73 means no power to Rc (cooling power), while E74 means no power to Rh (heating power). Both indicate missing 24V AC power to your thermostat.
Will this damage my HVAC system? The float switch and fuse are safety devices designed to protect your system. A tripped switch prevents water damage, while a blown fuse prevents electrical damage from short circuits.