What does error code AO mean on a Hayward pool heater?

Blower vacuum switch fault. The blower vacuum switch is open (AO) or closed when it shouldn't be (AC).

Can I fix Hayward code AO myself?

Start with these safe checks: Check from outside that nothing is blocking the heater's air openings or vent. Blower and switch checks are combustion work for a qualified tech.

When should I call a technician for code AO?

Blower and switch checks are combustion work for a qualified tech. Smell gas? Leave right away, don't touch light switches or phones inside, and call your gas utility or 911 from outside. Never open a gas valve, burner, gas line, vent or flue connection, or bypass a safety switch. Repeated lockouts, scorching, soot, or a CO alarm mean shut it off and call a licensed gas technician.

Hayward Pool Heater Error Code AO / AC: Blower vacuum switch fault

What AO means

The blower vacuum switch is open (AO) or closed when it shouldn't be (AC).

Also shown as: AC, depending on the model.

Likely causes

Blower, vacuum switch or tubing fault
Blocked air

Safe first checks

Check from outside that nothing is blocking the heater's air openings or vent.

When to stop

Blower and switch checks are combustion work for a qualified tech.

Checked against: Hayward Universal H-Series heater troubleshooting guide (UHS16b): diagnostic codes. Codes can differ by model and year, so confirm with your owner's manual.