Thermostat calls for heat but the furnace does not start; no error code is shown on the thermostat.

Safety first: Turn off power to the furnace/air handler at the breaker before moving wires.

• Step 1 – Verify thermostat settings: Confirm the thermostat is in Heat mode and the setpoint is above the current room temperature.
• Step 2 – Inspect W wiring: Remove the thermostat and ensure the W (or W1) wire is secure in the W terminal. Check the same wire at the furnace control board.
• Step 3 – Test furnace with jumper: With power on at the furnace, briefly jumper R to W at the control board. If the furnace starts, the issue is in the thermostat or wiring.
• Step 4 – Continuity test: With power off and W disconnected at both ends, test continuity of the W conductor. Use a spare conductor if the original is open.
• Step 5 – Reset furnace lockout: Some furnaces enter lockout after repeated failed ignition. Turn power off to the furnace for 5 minutes, then restore power and retry.
• Step 6 – Replace thermostat: If wiring and furnace are good, the W relay on the Geo thermostat may be defective. Replace the thermostat head unit.