On the covered Google Nest thermostat, E3, E23, E73, E195 means the thermostat detects no power to the Rc wire. Google's table words E195 as no power to the R wire. First, check that the breakers and the system power switch are on, especially if construction or maintenance was done recently.
Try in this order
0 of 9 completedWhat to do first
ReversibleCheck that the breakers and the system power switch are on, especially if construction or maintenance was done recently.
Correct only the documented fault domain
ReversibleGoogle says that if this code appears in the summer you will likely need an HVAC professional, because a struggling air conditioner can stop powering the thermostat. Its hot-weather steps: check the thermostat wiring, replace a clogged air filter, check the drip pan and drain tube (a backed-up drain can shut the system off), and inspect the HVAC fuse on the furnace control board, replacing it if it is blackened or its filament is broken. If your thermostat will not turn on, remove the display and charge it with a USB cable for at least 10 minutes before you read the code.
Confirm the transformer has power
ReversibleE3 type codes mean the Rc wire is connected but carries no power. Check the furnace switch, the door switch and the breaker before the thermostat.
Test the Rc wire
ReversibleWith the breaker off, disconnect the Rc wire only, then turn power on and measure the voltage on it; 24 V AC means the wiring is sound.
Check the system's power switch and breaker
ReversibleGoogle's first step is to confirm that the breakers and the system power switch are on, especially if building work or maintenance was done recently. A furnace or air handler that has been switched off at its own switch stops powering the thermostat through the Rc wire.
Charge the thermostat before reading the code
ReversibleIf the thermostat will not turn on, Google says to remove the display and charge it with a USB cable for at least ten minutes before reading the code. A flat battery can look like a power fault.
In warm weather, look at the air conditioner
ReversibleGoogle notes that a struggling air conditioner can stop powering the thermostat, so a code in summer may need an HVAC professional. Check that the air filter is not clogged, since a blocked filter can make the system shut down.
Check the drip pan and the drain tube
ReversibleA backed-up condensate drain can shut the system off through a float switch, which also cuts the thermostat's power. Look for water in the drip pan and a blocked tube, and call a professional if there is standing water.
Have the HVAC fuse inspected
ReversibleGoogle lists the HVAC fuse on the furnace control board, which should be replaced if it is blackened or its filament is broken. Opening the furnace carries risk, so a technician should do it, and the thermostat wiring should be checked at the same time.
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Exact questions
Common questions about “Google Nest thermostat code E3, E23, E73, E195”
What does E3, E23, E73, E195 mean on a Google Nest thermostat?+
The thermostat detects no power to the Rc wire. Google's table words E195 as no power to the R wire.
Why does it appear in summer?+
A struggling air conditioner can stop powering the thermostat.
Can a flat battery cause it?+
It can look like it, so charge the thermostat first.
Do I need an HVAC professional?+
Often yes, especially in hot weather or if a fuse is involved.