Furnace displays a flow switch or flow sensor error; burner will not fire for heating or DHW.

Safety first: Isolate power and close water valves. Have towels ready for minor water spills.

• Check system pressure: Ensure the heating circuit is correctly pressurized; very low pressure can prevent flow.
• Clean inlet filter: Many Sime hydraulic groups include a mesh filter on the cold water or return inlet. Close valves, remove the filter plug, and clean the mesh under running water.
• Inspect flow sensor: On models with a turbine flow sensor, remove the sensor body and check that the turbine spins freely. Rinse out debris and reinstall with new O‑rings if needed.
• Check wiring: Verify that the flow switch/sensor connector is firmly attached to the control board.
• If the error persists: The flow sensor or hydraulic group may need replacement by a technician.