F9 displayed, appliance locks out, may be associated with very high temperature readings

Safety first: Isolate power and gas. Let the unit cool. Overtemperature conditions can damage components.

• Check NTC readings: Compare flow and return NTC values in the diagnostic menu (if available). If one is unrealistic, test and replace that sensor.
• Verify circulation: On models connected to a storage tank or heating circuit, ensure the circulation pump is running and valves are open.
• Inspect for scale: If the unit is used in hard water areas, suspect scale in the primary or plate heat exchanger. Perform descaling as per Protherm instructions.
• Reset lockout: After correcting issues, reset the lockout via the front panel or by cycling power. Monitor operation to ensure temperatures remain within normal range.

Warning: Persistent overtemperature faults without clear cause require professional diagnosis to avoid heat exchanger damage or unsafe operation.