i get an e195 error on my nest and now it won't control the heat

Diagnosis

The E195 error on the Google Nest Learning Thermostat (4th Gen) is a critical power failure notification specifically targeting the Rh (Heating Power) wire. This error indicates that the thermostat is not detecting the required 24V AC voltage from your heating system's transformer. Because the thermostat relies on this constant stream of power to trigger the heat relay, the system will completely refuse to engage the furnace or boiler as a safety precaution to prevent hardware damage. This failure usually stems from one of three root causes: a tripped safety switch on the furnace, a blown 3-amp or 5-amp fuse on the HVAC control board, or a physical break/disconnection in the Rh wire. You can confirm this by checking the thermostat display; if the E195 code persists even after a restart, the issue is external to the Nest software and exists within the electrical path between the furnace control board and the wall base. If you have a multimeter, you can confirm this by testing for 24V AC between the Rh and C wires at the base.

How to Fix It

Follow these steps in exact order to restore power to your Rh terminal:

1. Shut off the power to your HVAC system at the main electrical breaker panel to prevent short-circuiting the control board.
2. Locate the furnace or air handler unit and ensure the local power toggle switch (usually a light switch located on the side of the unit or on a nearby wall) is flipped to the ON position.
3. Remove the furnace access panel to expose the control board. Locate the automotive-style blade fuse (typically a purple 3A or amber 5A fuse) plugged directly into the board. Pull the fuse and inspect the metal filament; if it is broken or the glass is charred, replace it with an identical amperage fuse.
4. Inspect the wiring at the furnace control board. Locate the terminal labeled 'R' or 'Rh'. Ensure the wire is stripped back about 1/4 inch and is clamped tightly under the screw terminal. Tug the wire gently to ensure it doesn't slide out.
5. Return to the Nest Thermostat. Pull the display off the base and inspect the Rh connector. Ensure the copper wire is fully inserted into the connector and that the button on the terminal is depressed and locked. If the wire is bent or frayed, trim it and strip a fresh 1/4 inch of insulation.
6. Restore power at the breaker and wait 2-5 minutes for the Nest to reboot and re-scan the voltage.

If That Didn't Work

• Check the condensate overflow switch: If you have a high-efficiency furnace or an AC unit integrated into the system, check the drain pan. If the pan is full of water, the float switch will trip, cutting power to the Rh wire to prevent a flood. Clear the drain line using a wet/dry vac to reset the switch.
• Inspect for wire corrosion: If your thermostat wiring runs through an attic or crawlspace, look for signs of rodent damage or corrosion. A partial break in the Rh wire can cause intermittent E195 errors. If the wire is damaged, you will need to fish a new 18/5 or 18/8 thermostat cable through the wall.
• Test the Transformer: Use a multimeter set to AC voltage to test the transformer output at the furnace. If you see 0V or significantly less than 24V (e.g., 12V), the transformer itself has failed and must be replaced (typically a 40VA transformer).
• Verify C-Wire Stability: While E195 is an Rh error, a lack of a Common (C) wire can cause power fluctuations that trigger false E195 readings. If you are relying on "power stealing," install a Nest Power Connector at the furnace to provide a dedicated return path for the current.

Heads Up

• Never replace a blown furnace fuse with one of a higher amperage; doing so can permanently fry the expensive integrated control board.
• Always ensure the thermostat display is fully seated on the base; a loose connection can mimic a power failure.
• If you recently installed a new air filter, ensure it is seated correctly; some modern furnaces have "filter pressure switches" that kill power to the R terminal if the filter is missing or incorrectly installed.
• Avoid using electrical tape to secure wires at the Nest base, as the heat from the terminals can cause the adhesive to melt and create a short.