Safety first: Turn off power and gas. Venting issues can lead to carbon monoxide hazards.

Steps to address AO error:

Inspect exhaust vent: From the heater to the termination, check for obstructions, disconnected joints, or crushed sections. Clear any debris.
Check combustion air intake: For models with dedicated intake, ensure the inlet screen is clean and not blocked by leaves, insects, or snow.
Verify blower performance: Listen for unusual noises, grinding, or low airflow. Replace the blower assembly if it is weak or not reaching full speed.
Inspect vent/air pressure switch: Check tubing and wiring to the vent pressure switch. Test the switch with a manometer or by applying gentle pressure/vacuum to confirm operation.
Reset and test: After clearing any obstructions and verifying components, restore power and gas and test operation. If AO returns, further diagnostic by a technician is recommended.

Warning: Do not operate the heater with known venting or air supply problems.

Display shows 'AO' and heater will not fire; may indicate air flow or venting issue on some Raypak models.

Possible Causes