Boiler displays E4 and shuts down; may occur after running for a short period

Safety first: Turn off power and allow the boiler to cool before touching any piping or sensors.

• Check system pressure and air: Look at the boiler pressure gauge. If pressure is very low (often below 12 psi in many systems), air pockets can cause overheating. Bleed air from radiators or air vents and restore pressure per the system design.
• Verify all valves are open: Ensure isolation valves on the boiler supply and return are fully open. Check zone valves and circulator isolation valves.
• Confirm circulator pump operation: When there is a call for heat, feel the circulator pump body. It should vibrate slightly and get warm. If it stays cold and silent, the pump or its relay may have failed.
• Inspect high limit sensor wiring: Locate the high limit/temperature sensor on the supply outlet. Check that its wiring is intact and firmly connected to the control board.
• Reset and monitor: After correcting flow issues, reset the boiler and monitor temperature. If E4 recurs quickly, the high limit sensor or control board may need replacement and the heat exchanger should be checked for scale by a technician.