Display shows 'bP' or similar and unit indicates bypass mode

Safety first: No electrical isolation is required, but do not open pressurized components.

• Check bypass valve: Inspect the Viessmann bypass assembly. Ensure the handle or knobs are set to the 'Service' position according to the label.
• Confirm display: The controller should show that the unit is in service mode once the valve is correctly positioned.
• Inspect actuator (if motorized): For models with an automatic bypass, verify that the actuator is connected and moves when commanded.
• Sensor check: If the valve is in service but 'bP' remains, the bypass position sensor may be faulty and require replacement.