5 flashes = Vacuum switch / draft pressure issue.
Atmospheric units — inadequate draft
- Chimney obstruction (bird nest, debris, ice)
- Improper chimney height — code requires 3 ft above roof + 2 ft horizontal clearance
- Liner damage
- Wind effects
- Negative pressure in home (kitchen exhaust, dryer)
Power Vent units — pressure switch
- Switch failed (open or shorted)
- Tubing kinked or disconnected
- Vent obstruction (PVC sidewall blocked)
- Fan not running
Reset
- Address the obstruction or fault
- Cycle gas valve
- For power vent: cycle 120V power
- Monitor LED
Bottom line
Atmospheric: inspect chimney. Power vent: pressure switch tubing and vent termination.