Air pressure switch contacts are open.
- Check the wiring connections to switch. Wires should be connected to the common and normally closed terminals.
- Air intake lengths exceed the maximum allowed lengths.
- Check for obstruction or blockage in the air intake pipes or at terminations.
- Check reference hoses connected to the air pressure switch for blockage or obstruction.
- Inspect the burner. Replace if necessary.
- Inspect the heat exchanger.
- Faulty air pressure switch. Replace switch.
