Nest thermostat shows E4 and heating will not engage

Safety warning: Turn off power to the furnace/boiler at the breaker or service switch before working on wiring.

• Inspect W1 at Nest: Remove the Nest display and check that the W1 wire is straight, clean, and fully inserted into the W1 terminal. Re-strip if necessary.
• Check W at equipment: At the furnace or boiler control board, verify the thermostat W/W1 wire is under the correct terminal and the screw is tight.
• Look for cable damage: Follow the thermostat cable as far as accessible and check for staples through the cable, crushed sections, or exposed copper. Repair or replace as needed.
• Test heat relay (advanced): With power on and thermostat calling for heat, a technician can measure 24 VAC between W and C at the control board. If voltage is present but the furnace does not start, the board or safety chain may be at fault.

If you are not comfortable testing live voltages, contact an HVAC professional to diagnose the W1 circuit and control board.