Oso water heater with integrated circulation pump shows E09 and stops heating.

Safety first: Isolate electrical power and allow the system to cool before working on the pump or pipework. Close isolation valves where fitted.

• Listen for pump operation: When power is on and a call for heat is present, you should hear or feel the pump running. If it is silent, it may be seized or not powered.
• Bleed air: Use any bleed screw on the pump body or nearby high points to release trapped air. Open a hot tap to assist flow while bleeding.
• Check valves and filters: Inspect any inline strainers or check valves on the Oso cylinder’s primary circuit for blockage. Clean or replace as necessary.
• Test pump electrically: With power off, disconnect the pump and measure winding resistance. If open or shorted, replace the pump. Also verify that the control board is sending power to the pump when required.
• Flow sensor: If fitted, inspect the flow sensor wiring and clean the sensor body of debris. Replace if faulty.

If E09 persists after restoring flow and pump operation, the control PCB may not be recognizing the feedback and should be checked by a technician.