F1 displayed; heater does not start or stops shortly after starting.

Safety: Close the cold water inlet and relieve pressure by opening a hot tap before removing any water components.

• Clean inlet filter: At the cold water inlet of many Junkers heaters there is a mesh filter. Remove and rinse under running water to clear sand or debris.
• Check flow sensor: If accessible, remove the flow sensor/turbine assembly. Inspect for limescale or dirt preventing rotation. Clean gently with vinegar solution and a soft brush.
• Verify orientation: Ensure the flow sensor is installed in the correct flow direction (arrow on body).
• Reassemble and test: Refit parts, open inlet valve slowly, check for leaks, then test hot water. If F1 persists, the sensor may need replacement.