Heater displays E10 and refuses to start a heating cycle.

Safety: Turn off power before opening the heater.

• Verify water flow: Ensure the pump is running at the correct speed (for variable-speed pumps), filter is clean, and valves are open. Low flow can cause excessive temperature rise across the heater.
• Compare sensor readings: Using the heater’s diagnostic menu (if available), compare inlet and outlet temperatures with an external thermometer in the plumbing. A large discrepancy may indicate a bad sensor.
• Inspect for scale: If flow is good but the outlet temperature is much higher than inlet, the heat exchanger may be scaled or restricted. This requires professional descaling or replacement.
• Replace faulty sensor: If one sensor clearly reads incorrectly compared to actual water temperature, replace that sensor.