Thermostat displays W002 and the heating system runs continuously or cycles unexpectedly even when GLAS is not calling for heat.

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

• Step 1 – Inspect GLAS wall plate: Remove the thermostat and check that no bare copper from the W wire is touching the R terminal or other conductors. Trim and re‑terminate if necessary.
• Step 2 – Inspect furnace terminals: At the control board, verify that W and R wires are not touching and that no stray strands are bridging terminals.
• Step 3 – Cable inspection: If accessible, inspect the thermostat cable along its route for crushed or damaged sections that could cause a short.
• Step 4 – Isolate GLAS: Disconnect the W wire from the GLAS wall plate and restore power. If the furnace still runs, the short is in the wiring or control board. If the furnace stops, reconnect W and test again; if the problem returns only when GLAS is connected, suspect a stuck W relay.
• Step 5 – Replace faulty component: Repair or replace damaged thermostat cable or furnace control board as needed. If the GLAS W relay is stuck closed, replace the thermostat.