Boiler shows A21; burner stops and may not restart

Safety: Isolate electrical power and close water isolation valves before removing any hydraulic components.

• 1. Confirm symptom: On combi models, open a hot water tap and see if the boiler recognizes flow (DHW symbol on display). If not, A21 may be due to a faulty DHW flow sensor.
• 2. Clean inlet filter: Many Buderus combi boilers have a cold water inlet filter. Close the cold inlet valve, remove the filter, and clean any debris or scale.
• 3. Inspect flow sensor: Remove the DHW flow sensor assembly (often a small plastic housing with a turbine). Check for limescale or debris preventing rotation. Clean carefully and reassemble with new O-rings if needed.
• 4. Electrical test: With the sensor connected and power on, a technician can measure the sensor signal (reed switch pulses or Hall sensor output) while water flows. If no signal, replace the flow sensor.
• 5. Wiring and connector: Check the low-voltage cable from the flow sensor to the control board for damage or loose pins.
• 6. System flow (if applicable): On some systems, A21 may refer to a system flow switch. Ensure all valves are open and the circulation pump is operating.