Thermostat displays F.03 and cannot detect room temperature correctly

Safety: Isolate mains power to the boiler and any wiring center before opening covers. Only work on low‑voltage eBus terminals; do not touch mains terminals.

Diagnosis:

• Confirm whether an external room sensor (Glow‑worm eBus room sensor or outdoor sensor) is actually installed. If not, check the thermostat configuration to ensure no external sensor is enabled in the installer menu.
• If an external sensor is installed, locate it (usually a small wall‑mounted sensor) and inspect for physical damage or water ingress.
• Check the two‑core cable from the sensor back to the boiler or wiring center. Look for crushed insulation, rodent damage, or loose terminal screws.
• With power isolated, remove the sensor from its backplate and measure resistance across the sensor terminals with a multimeter. Compare to Glow‑worm’s NTC resistance chart (e.g. around 10 kΩ at 25 °C). Infinite or near‑zero resistance indicates a failed sensor or shorted cable.

Remedy:

• Repair or replace damaged cable runs between the sensor and boiler eBus terminals.
• If the sensor resistance is out of specification, replace the external Glow‑worm NTC room sensor with the correct part.
• In the thermostat’s installer menu, ensure the correct sensor type is selected and that external sensor mode is enabled only if a sensor is present.
• Restore power and verify that F.03 clears and that room temperature readings are stable.