Heater shows E09 and shuts down; may occur when set temperature is high and flow is low.

Safety: Turn off power and allow the unit to cool before working on it.

• Reduce set temperature: Lower the setpoint on the control panel to a moderate value (e.g., 120°F / 49°C) and test again.
• Increase flow: Fully open the hot water tap and avoid mixing with cold at the fixture while testing. Low flow with high set temperature can cause rapid overheating.
• Clean inlet filter and aerators: Remove restrictions that may be limiting flow as described for E08.
• Check for scale: If the unit is in a hard water area and has not been descaled, consider performing a descaling procedure on the heat exchanger using a pump and descaling solution per the service manual.
• Reset and test: After addressing flow and scale, restore power and test. If E09 persists at moderate temperatures and normal flow, a faulty sensor or control board may be causing improper temperature regulation and should be diagnosed by a technician.