i get error e195 on my nest, something about power sharing? my heat won't run.

Diagnosis

The E195 error on a Google Nest Learning Thermostat (3rd Gen) is a critical safety trigger related to "Power Sharing." This occurs when the thermostat detects an unexpected voltage drop or an irregular current draw on the Rh or Rc wire while trying to engage the heating system. Essentially, the Nest is sensing that it cannot pull enough steady power to maintain the connection to your HVAC control board, leading it to shut down the call for heat to prevent damage to the internal relays or the furnace control board. This typically happens in systems lacking a dedicated Common (C) wire. Without a C-wire, the Nest "steals" power from the heating or cooling circuits. If your HVAC system has a sensitive control board or a high-resistance circuit, the "power sharing" mechanism fails, causing the voltage to dip below the operational threshold. You can confirm this by checking the "Power" section in the Technical Info menu on the device; if the Voc or Vin readings are fluctuating significantly or are below 3.6V, the thermostat is starving for power, triggering the E195 lockout.

How to Fix It

To resolve the E195 error, you must stabilize the power delivery to the thermostat. The most permanent and reliable fix is installing a C-wire or a Power Connector.

1. Check the Wiring: Remove the Nest display from the base. Ensure the Rh or Rc wire is seated deeply in the connector and that no copper strands are frayed or touching adjacent terminals.
2. Verify System Configuration: Navigate to Settings > Equipment on the device. If you have a heat pump, verify the O/B setting. For Rheem or Ruud systems, this must be set to "B"; for almost all other brands (Carrier, Trane, Lennox), it must be set to "O". An incorrect reversing valve setting can cause the system to draw power incorrectly.
3. Install a Nest Power Connector: If you do not have a C-wire, purchase the official Google Nest Power Connector (Part #G9S1K). Install this at the furnace/air handler control board. Connect the connector's wires to the R and C terminals on the board, and then bridge the existing thermostat wires into the connector. This provides a dedicated return path for power without needing to fish a new wire through your walls.
4. Perform a Power Cycle: Once the hardware is adjusted, flip the breaker for your HVAC system OFF for 60 seconds, then flip it back ON. This clears the E195 error code from the thermostat's volatile memory.

If That Didn't Work

• Install a 24V AC Transformer: If you cannot access your furnace board, buy a 24V AC plug-in transformer (VA rated at 40VA). Connect the two wires from the transformer to the C and Rc terminals on the Nest base. This bypasses the HVAC system's power supply entirely for the thermostat's electronics.
• Check the HVAC Fuse: Inspect the 3-amp or 5-amp blade fuse on your furnace control board. A partially blown or pitted fuse can cause voltage drops that trigger the E195 error even if the system seems to be working intermittently.
• Test the Control Board: Use a multimeter to measure the voltage between R and C at the furnace. You should see a steady 24V-28V AC. If the voltage drops significantly when the thermostat calls for heat, your transformer may be undersized or failing, requiring a replacement of the HVAC transformer.
• Factory Reset: Go to Settings > Reset > All Settings. Occasionally, a corrupted software update can cause the power-sensing algorithm to malfunction. Resetting and re-configuring the equipment from scratch can clear the logic error.

Heads Up

• Avoid "Power Stealing": While Nest claims to work without a C-wire, 3rd Gen models are notorious for causing "chattering" contactors or E195 errors in high-efficiency furnaces. Always prioritize a C-wire installation.
• Wire Gauge Warning: Ensure you are using 18-gauge thermostat wire. Using thinner bell wire or old circuitry can increase resistance, leading to voltage drops and power sharing errors.
• Battery Maintenance: If the device has been powered off for a long time, the internal lithium-ion battery may be depleted. Let the device charge via the USB port on the back of the display for 30 minutes before re-mounting it to the base.