Pool Error Codes

Hayward AO: Blower Vacuum Switch Open

An AO code means the heater's combustion blower never proved it was moving air the way the control board expects before it will allow ignition.

Medium severity

Applies to: Hayward Universal H-Series and H-Series residential gas pool/spa heaters

What it means

Before an H-Series heater ever opens the gas valve, its startup sequence checks the blower vacuum switch twice: once to confirm it's open at rest, and again about 20 seconds later, after the blower has spun up and the igniter has been pre-heating, to confirm the switch has closed under real airflow. AO fires when that second check fails — the board never sees confirmation that combustion air is actually moving through the heater and out the vent, so it won't risk lighting the burner. Because the switch itself is simple and rarely the point of failure, AO usually comes down to something physically blocking air on either side of that path: the vent outside, the intake, or the small sensing tubing that connects the switch to the airflow it's measuring.

The timing matters too: the pre-heat and airflow check both happen inside a roughly 20-second pre-purge window before the burner ever gets a chance to light, and the blower keeps running for another 30 seconds after each heating cycle ends to clear any residual combustion byproducts from the flue. That post-purge step is part of why AO can sometimes appear seemingly out of nowhere on a heater that was working minutes earlier — a partial vent obstruction that was borderline during pre-purge can tip into a full failure once conditions outside shift slightly, whether that's wind direction, a bird investigating the vent cap, or debris settling after a windy afternoon.

Likely causes

What you can check yourself

  1. Visually check the vent and combustion air intake outside the heater for blockages and clear any debris
  2. Confirm the blower spins freely and sounds normal when the heater attempts to start
  3. Power-cycle the heater after clearing any obvious obstruction
Call a professional if…

Don't disconnect or bypass the vacuum switch tubing to force a start. If the vent is clear and the blower runs but AO persists, the switch or its tubing connections need professional testing.

Sources

Related

This site is an independent reference, not a substitute for a licensed pool professional. Gas pool heaters can leak carbon monoxide and cause fire; pump and heater electrical circuits carry mains voltage. Anything involving the gas line, burner assembly, or house wiring should be handled by a licensed technician or electrician.