Thermostat or boiler display shows 'Comm Err' or communication error; setpoint changes on thermostat do not reach boiler.

Safety first: Isolate mains power to the boiler at the fused spur before touching any wiring. Do not remove sealed combustion covers; only access the external wiring centre or interface terminals.

• Check BUS wiring: Locate the Worcester Bosch BUS/EMS terminals on the boiler or wiring centre and on the thermostat interface. Ensure the two‑core cable is firmly connected and not damaged.
• Polarity: Some Worcester Bosch BUS connections are polarity sensitive. Verify that the same conductor goes to the same terminal at both ends (e.g. BUS+ to BUS+, BUS− to BUS−).
• Inspect for damage: Look for crushed, cut, or water‑damaged cable between thermostat and boiler.
• Power cycle: After checking wiring, restore power and allow the boiler and thermostat to reboot. Many communication errors clear after a clean restart.
• If error persists: The boiler interface PCB or thermostat interface module may be faulty. A Gas Safe or qualified heating engineer should test and replace these components.