Thermostat displays C5 and indicates that the domestic hot water circulation pump control is faulty.

Safety first: Turn off power to the boiler and pump circuits. Hot water pipes may be very hot.

• Check pump schedule: In the thermostat or boiler menu, verify that the DHW circulation schedule is correctly set and not disabled.
• Inspect pump wiring: Ensure the DHW circulation pump is wired to the correct output terminals and that connections are tight.
• Listen for pump operation: During a scheduled run, listen or feel for pump vibration. If silent, the pump or output may be faulty.
• Test pump directly (technician): A professional can test the pump by powering it directly to confirm if it runs.
• Replace pump or repair output: If the pump is defective, replace it with a compatible Viessmann‑approved model. If the controller output is damaged, the control board may need replacement.