i keep getting an e195 error on my nest and my heat won't turn on, what does that mean

Diagnosis

The E195 error on the Google Nest Learning Thermostat (4th Gen) is a critical power failure notification specifically targeting the Rh wire, which is the 24VAC power feed from your heating system. Essentially, the thermostat is telling you that it is not detecting the necessary voltage required to trigger the heating relay. This creates a "dead loop" where the thermostat cannot send a signal to the furnace because it has no power to send, leaving your home cold. This issue typically stems from one of three root causes: a physical disconnection (a loose wire), a tripped safety mechanism in the HVAC unit, or a failed transformer. To confirm this is a power delivery issue rather than a software glitch, check the "Technical Info" menu under Settings on the Nest display; if the Rh voltage reads below 20V or shows as "Not Detected," the problem is external to the thermostat's software and exists within the physical wiring or the furnace hardware.

How to Fix It

Follow these steps precisely to restore power to your Rh terminal:

1. Shut off the power to your HVAC system at the breaker panel to avoid shorting the control board.
2. Gently pull the Nest display off the base. Inspect the red wire inserted into the Rh connector. Use a small flathead screwdriver or a needle to ensure the connector button is fully depressed and the wire is gripped tightly. If the wire is frayed, trim it back 1/4 inch and strip a fresh 3/8 inch of insulation.
3. Locate your furnace/air handler and open the main blower panel. Find the control board where the thermostat wires terminate. Locate the terminal labeled "R" or "Rh." Give the red wire a gentle tug to ensure it is seated firmly in the screw terminal.
4. Check your furnace's condensate overflow tray. If your system has a secondary drain pan and it is full of water, the float switch will physically break the Rh circuit to prevent a flood, triggering the E195 error. Clear the drain line using a wet/dry vac from the exterior exit point.
5. Restore power at the breaker and snap the Nest display back onto the base, ensuring the pins align perfectly.

If That Didn't Work

• Test the Transformer: Use a digital multimeter set to AC voltage. Place one probe on the Rh wire and the other on the C (Common) wire at the furnace board. You should see a reading between 24VAC and 28VAC. If the reading is 0V, your 24V step-down transformer has likely failed and needs replacement.
• Inspect the Fuse: Look for a small automotive-style blade fuse (usually 3A or 5A, purple or amber) located directly on the furnace control board. If the filament inside the fuse is broken, the Rh line is dead. Replace it with an identical amperage fuse; if it blows again immediately, you have a short circuit in your thermostat wiring.
• Verify the C-Wire: While E195 specifically mentions Rh, the 4th Gen Nest requires a stable return path. If you are relying on "power sharing" without a C-wire, the internal battery may have dropped below the threshold required to maintain the Rh connection. Install a Nest Power Connector or run a dedicated 18/2 thermostat wire to provide a constant 24V return.
• Check Zone Valve Controllers: If your home has multiple thermostats, locate the zone control panel. The E195 error often occurs when a specific zone transformer fails while others continue to work. Check for a "Fault" LED on the zone board corresponding to the room with the E195 error.

Heads Up

• Avoid "Jumping" Wires: Never attempt to jump the Rh and Rc terminals with a piece of wire to bypass the error; this can permanently damage the 4th Gen Nest's internal switching circuitry.
• Voltage Warning: While 24VAC is generally safe, always turn off the breaker before touching the furnace control board to prevent blowing the onboard fuse.
• Wire Gauge: Ensure you are using 18-gauge solid copper wire. Stranded wire can sometimes slip out of the Nest push-terminals, leading to intermittent E195 errors.
• Filter Maintenance: A severely clogged air filter can cause the furnace to overheat, tripping the high-limit switch. This switch cuts power to the Rh line to prevent a fire, which will manifest as an E195 error on your display.