Boiler shuts down with E04 and will not restart until reset

Safety: Allow the boiler to cool before touching any piping. Turn off power before removing covers.

• Check system pressure and air: Look at the boiler pressure gauge. If pressure is very low or zero, the system may be air-bound or underfilled. Do not add water unless you understand your system; contact a technician if unsure.
• Verify circulator pump operation: When there is a call for heat, listen and feel for vibration at the Dettson system circulator pump. If it is silent and cold, check the breaker and service switch. A technician may need to replace the pump or pump relay.
• Bleed air from radiators: If some zones are cold and you hear gurgling, air may be trapped. Carefully bleed air from radiators or baseboards using bleed valves, watching for water. Protect floors from spills.
• Inspect high limit sensor: Locate the Dettson high limit temperature sensor on the supply pipe or heat exchanger. Ensure it is firmly attached and wiring is intact. A failed sensor can falsely trip E04 and must be replaced.
• Reset and monitor: After correcting circulation issues, reset the boiler and monitor temperature. If E04 recurs quickly, stop using the boiler and call a professional to check flow, sizing, and sensor calibration.