Smart thermostat shows F.04 and cannot communicate with the boiler

Safety: Isolate mains power to the boiler before removing the front panel or accessing the wiring terminals. eBus is low voltage but is inside a mains appliance.

Checks:

• Confirm that the thermostat is powered and its display is active. If the thermostat is battery powered, replace the batteries first.
• At the boiler, locate the eBus terminals (often marked eBUS, +/- or BUS). Ensure the two‑core cable from the thermostat interface is firmly connected and not mixed with 230 V terminals.
• Inspect the cable for damage, kinks, or signs of overheating. Repair or replace any damaged section.
• Verify polarity if required by the specific Glow‑worm control (some are polarity‑insensitive, but check the manual). Correct any reversed connections.
• If a wiring center is used, check all intermediate connections and junctions for loose screws or corrosion.

Remedy:

• Re‑terminate any loose or corroded eBus connections using appropriate ferrules where necessary.
• If the thermostat uses a separate eBus interface module (e.g. Glow‑worm internet gateway or system controller), power‑cycle that module and check its status LEDs.
• If F.04 persists and wiring is sound, the eBus interface module or boiler control PCB may be faulty. At this point, contact a Glow‑worm service engineer for further diagnostics and replacement.