Heater shows AO and will not fire; blower may not start or may run briefly.

Safety first: Turn off power and allow the heater to cool. Do not operate with the cabinet open.

• Check for obstructions: Inspect the exhaust vent and air intake for nests, leaves, or debris. Clear any blockage.
• Inspect blower: Remove the front panel and verify the blower wheel spins freely by hand (with power off). Look for signs of water intrusion or rust.
• Check air tubing: Follow the small rubber or silicone tube from the blower housing to the air pressure switch. Ensure it is not cracked, kinked, or disconnected. Replace if brittle.
• Test pressure switch (basic): With power off, check that the switch terminals are firmly connected. If the heater runs when the switch is temporarily jumped (for testing only by a professional), the switch is likely bad and should be replaced.
• If AO remains: The blower motor or control board may be defective. Contact a qualified technician for further diagnosis.