E5 error when calling for heat; furnace or heat pump does not start

Safety: Turn off power to the furnace/air handler at the breaker before inspecting wiring.

• Check thermostat W terminal: Remove the thermostat from the wall plate and verify the W (or W1) wire is fully inserted and clamped.
• Inspect furnace board: At the furnace, locate the W terminal and ensure the thermostat cable is secure and not corroded or broken.
• Fuse check: Inspect the low-voltage fuse on the furnace control board. Replace if blown, but look for shorts in thermostat wiring first.
• Wiring inspection: Follow the thermostat cable from furnace to wall. Look for staples through the cable, rodent damage, or crushed sections that could short W to other conductors.
• Test after reset: Restore power, set the thermostat to Heat and raise the setpoint. If E5 reappears and the furnace does not start, the thermostat’s W output or furnace control board may be defective.
• Resolution: Replace the thermostat if wiring and fuse are good. If issue persists, have a technician test the furnace control board and safety circuits.