Boiler shuts down with E04; display may indicate high limit or overheat; unit is hot to the touch.

Safety: Allow the boiler to cool before touching piping. Turn off power before working on wiring or sensors.

• Check system valves: Ensure all supply and return isolation valves near the IBC boiler are fully open. Partially closed valves can cause localized overheating.
• Verify circulation: During a call for heat, feel the supply and return pipes. If the supply is very hot and the return is cool, the system pump may not be running or there may be an airlock. Listen for pump operation and vibration.
• Bleed air: If your system has air vents or purge valves, carefully bleed air from high points and near the boiler. Follow the hydronic system's purge procedure.
• Inspect high limit sensor: Locate the high limit/temperature sensor on the boiler heat exchanger. Check that it is firmly attached and its wiring harness is secure.
• Reset: After the boiler cools, reset the lockout from the control panel and monitor operation.

If E04 recurs, a technician should test the system pump, verify flow rates, and replace the high limit sensor or pump as needed.