Nest thermostat shows E10 and reports that it cannot detect the R wire or power from the HVAC system

Safety: Turn off power to the HVAC system before opening panels. Use caution when working around electrical components.

• Check R wire at Nest: Remove the Nest display and confirm the R wire is fully inserted and clamped. Re‑strip if corroded or damaged.
• Inspect furnace control board: Verify the thermostat R conductor is attached to the R terminal. Tighten the screw if loose.
• Fuse and transformer: Inspect the low‑voltage fuse and replace if blown. If the fuse is intact but there is no 24 VAC between R and C at the control board, the 24V transformer may have failed and will need replacement by a technician.
• Continuity test: With power off, test continuity of the R conductor between the furnace and thermostat using a multimeter. If open, run a new thermostat cable or use a spare conductor.
• Base damage: If 24 VAC is present at the furnace and at the thermostat cable but not detected by the Nest, the Nest base R terminal may be damaged; replacing the thermostat is usually required.