Thermostat shows E03 and will not control heating; system remains off even when heat is requested.

Safety first: Turn off power to the HVAC system at the breaker before removing covers or moving wires.

• Step 1 – Inspect thermostat connections: Remove the Geo thermostat and check that the W (or W1) wire is fully inserted and clamped in the W terminal on the wall plate. Tighten if loose.
• Step 2 – Inspect furnace connections: At the furnace/air handler control board, verify the W wire is firmly attached to the W terminal and not corroded or broken.
• Step 3 – Continuity test: With power off and both ends of the W wire disconnected, use a multimeter in continuity mode between the W conductor at the thermostat end and the W conductor at the furnace end. If open, the cable is damaged and a spare conductor should be used or the cable replaced.
• Step 4 – Check for shorts: Inspect the thermostat cable along its run (if accessible) for staples or sharp bends that may have pierced the insulation. A short between W and C or W and R can trigger E03.
• Step 5 – Test furnace response: With power on, temporarily jumper R to W at the furnace control board. If the heat starts, the furnace is fine and the issue is in the thermostat or wiring. Remove the jumper after testing.
• Step 6 – Replace thermostat head: If wiring checks out and the furnace responds to an R‑W jumper, the W relay on the Geo thermostat PCB may be failed. Replace the thermostat head unit.