Quick answer
Error E195 on a Nest thermostat means no power to the R wire was detected. Without R power the thermostat cannot call for heating or cooling, and it will eventually shut down on a low battery. Check the breaker and the furnace switch, then confirm the R wire is fully seated in the thermostat base. If power is on and the wire is seated, call a technician.
When a Nest shows E195, the thermostat is usually not the broken part. Error E195 on a Nest thermostat means no power to the R wire was detected, which is how Google's help-code table words it. The R wire carries 24-volt power from the furnace or air handler up to the thermostat, so the code is saying the power source downstairs has gone quiet. Google revises its help codes, and the message on the thermostat screen or in the app is the reference if it differs from this page.
What stops working with E195
Everything the thermostat controls. With no power on R, a Nest cannot make a heating or cooling call, so the house drifts toward the outdoor temperature. The display keeps going on its internal battery for a while, then the thermostat shuts down when that battery runs low. In January that makes E195 a no-heat call, not a gadget problem.
Check the breaker, the furnace switch and the R wire
- At the electrical panel, find the furnace or air handler breaker. If it has tripped, reset it once; if it trips again, leave it off.
- At the furnace, look for the switch that resembles a light switch on or near the cabinet. It gets turned off by accident, usually by someone reaching for the basement light.
- Make sure the furnace door is fully closed. A door switch cuts power to the control board, and the 24-volt supply with it, when the blower panel is not seated.
- Switch the breaker off, pull the Nest display off its base, and look at the R terminal. The wire should be straight and pushed all the way in, with the connector button staying down; reseat it if it slides out with a gentle tug.
- Restore power, press the display back on and give it a few minutes to recharge and recheck its wires.
Other Nest power codes have their own pages: Nest error E73 and Nest error E74.
Why the R wire loses power, in order of likelihood
- System power is off: a tripped breaker, the furnace switch, or a door panel left ajar after a filter change.
- A blown 24 V fuse on the furnace control board. It is a small automotive-style blade fuse, and it blows when a low-voltage wire shorts, which often happens during a thermostat install done with the power on.
- A condensate safety switch that has opened the R circuit. On many installs a float switch on the condensate pump or the air conditioner's drain pan is wired in series with R, so a full reservoir or a plugged drain cuts thermostat power on purpose to prevent a flood.
- A loose or broken R wire at the thermostat base, at the furnace board terminal, or at a splice in between.
Is it the Nest or the furnace?
With E195 it is nearly always the equipment side. A Nest that reports no R power is describing what it measures at its own terminal, and a new thermostat on the same dead wire shows the same thing. Before buying a replacement, look at the furnace: if the control board's status light is dark, the furnace has lost its own power and the thermostat is only the messenger.
If water is sitting in the condensate pump reservoir or around the base of the furnace, the safety switch has done its job and the drain is the repair. More on telling the two apart is in is it my thermostat or my furnace.
When to call, and what an E195 repair costs
Call when power is on, the wire is seated and E195 remains. Replacing the board fuse yourself is tempting, but a fuse that blows again has a short behind it that needs tracing: a thermostat cable stapled through its insulation, an outdoor unit wire nicked by a trimmer, or a failed coil in a contactor or humidifier valve. Fitting a larger fuse moves the damage to the transformer or the board.
- Diagnostic visit for a thermostat power fault: $90–$150, applied to the repair when you proceed.
- Condensate drain cleaning or trap replacement, when a safety switch cut the power: $150–$350.
- A new condensate pump, installed: $250–$450.
- A furnace control board, if a short has damaged it: $500–$1,200.
Tracing a low-voltage short is priced on the visit and confirmed in writing before the work starts.
The full table of Nest help codes is in our Nest thermostat error code guide. If the checks above did not bring the R wire back, ZK Mechanical's technicians handle furnace repair and thermostat wiring faults across Burlington, Oakville, Milton, Hamilton and Mississauga. Call (647) 801-1252 or tell us what the thermostat shows.