This display indicates that the flue temperature sensor of the boiler has failed. This is a serious safety issue. The boiler will not restart until the sensor is replaced by a technician and he pushes the RESET button on the display. This situation is indicated by the red light and the flashing word LOCKOUT on the display. During this lockout fault, the pump will be on as indicated on the second line of the display.
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 Supply Temperature Sensor chart in Figure 28 of this manual. The temperature on the chart should be close to the same as the temperature in the flue. If not, replace the flue sensor.
