F03 indicates the return 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.
- Check circulator pump operation.
- Ensure adequate flow through the boiler by accessing the status menu and determining there is less than a 50ºF rise from the return thermistor to the supply thermistor.
- Troubleshoot thermistor by following the steps in F02.
