F04 indicates that the flue temperature sensor of the boiler has failed. The boiler will not restart until a technician replaces the sensor and pushes RESET on the display. This is a serious safety issue as indicated by the illuminated red light and the word LOCKOUT flashing on the display. During this lockout fault, the pump will be on.
Inspect the flue sensor for physical damage or corrosion and replace it if necessary. Check the electrical connection to the flue sensor and repair as necessary. Measure the resistance of the sensor and refer to the sensor resistance table in this manual. The temperature on the chart should be close to the temperature in the flue. If not, replace the flue sensor.
