Thermostat shows F2 and cannot control hot water or heating

Safety first: Isolate mains power to the heating system and thermostat before touching any wiring.

Steps to diagnose:

• 1. Inspect external wiring: Remove the thermostat from its backplate. Check CH and HW output terminals for stray wire strands touching adjacent terminals or earth. Re‑strip and re‑terminate any suspect conductors.
• 2. Check connected loads: At the wiring centre, disconnect the CH/HW output from the thermostat. Measure resistance of the connected zone valve coil or boiler control input. A dead short (near 0 Ω) indicates a faulty valve coil or control board that must be replaced.
• 3. Test with outputs disconnected: With the thermostat re‑mounted but CH/HW wires temporarily removed and safely insulated, restore power. If F2 clears, the fault is external (wiring or valve). If F2 remains, the thermostat relay or PCB is likely damaged.
• 4. Inspect wiring centre: Look for signs of overheating, melted insulation, or carbon tracking between terminals. Replace any damaged wiring centre blocks.

When to call a professional: If a short is suspected inside the boiler or wiring centre, or if you are unsure how to safely isolate circuits, contact a qualified engineer.