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ErrBase

Hayward Pool Heater AO Error Code: Air Flow or Pressure Fault

Quick Answer

Hayward AO means the combustion air pressure switch stayed open during the startup sequence, blocking ignition as a safety measure. A blocked exhaust vent or air intake accounts for 40% of AO faults, most commonly a bird or debris nest in the flue. Combustion blower failure is the second most common cause.

Hayward AO means the combustion air pressure switch stayed open during the startup sequence, blocking ignition as a safety measure. A blocked exhaust vent or air intake accounts for 40% of AO faults, most commonly a bird or debris nest in the flue. Combustion blower failure is the second most common cause. The air pressure switch itself can also fail, but test it only after confirming the vent and blower are both functional.

HaywardPoolheaterSeverity: highDifficulty: intermediate76% DIY Success
Time to Fix
30–150 min
Difficulty
intermediate
Parts Cost
$20 – $220
Tools Needed
Multimeter, Flashlight
Wrong code?

Not seeing AO on your display?

Select what your appliance is actually doing and we'll take you to the right guide.

combustion air flow switch fault
PentairERR AFS

What Does the AO Code Mean?

AO is a combustion air flow fault - the pressure switch stayed open because the blower did not generate enough air pressure or the vent is blocked.

Most Likely Causes

Based on aggregated repair data, here is the probability breakdown for this error code:

Bird nest, leaves, or debris blocking the exhaust vent40%
Combustion blower motor failure or seized bearings24%
Failed combustion air pressure switch14%
Wiring fault between control board and blower or pressure switch12%
Blocked or collapsed intake air path10%

Symptoms You May Notice

  • AO displayed on panel at startup
  • Heater blower starts but shuts down before ignition attempt
  • Heater completely silent - blower not running
  • AO after winter when heater has been dormant

Can you reset a Hayward poolheater to clear the AO code?

After clearing the vent or replacing the blower or pressure switch, press Mode to standby, wait 30 seconds, then restart the heater. Allow the blower to reach full speed before the ignition attempt. If AO returns within the first 30 seconds, the blower is still not generating adequate pressure.

Tools Required for Diagnosis

MultimeterFlashlightScrewdriver set

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

1

Locate the exhaust flue outlet and the combustion air intake on

Locate the exhaust flue outlet and the combustion air intake on the heater. Visually inspect both for nests, leaves, insects, or physical damage. Use a flashlight to look deep into the flue.

2

Start a heat cycle and listen for the combustion blower to run

3

With power off, open the blower compartment

4

With the blower confirmed running at full speed

5

Inspect the rubber sensing tube that connects the blower housing

Replacement Parts

If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.

Part Name
Combustion blower motorCHXBLW001 · $100–$220
Air pressure switchCHXPRS002 · $20–$45

Repair vs. Replace Calculator

Appliance Age (Years)7 Yrs
Labor Approach
Our Verdict
REPAIR
Estimated Cost: $120 (Parts) vs $3000 (New Machine)

Frequently Asked Questions

What does AO mean on a Hayward pool heater?
AO is a combustion air flow or pressure fault. It means the combustion air pressure switch did not confirm adequate air flow to the burner before ignition was attempted. The heater locks out to prevent ignition without proper combustion air, which would cause incomplete combustion and carbon monoxide production.
Can I fix Hayward AO myself?
Yes, if the cause is a vent blockage or a cracked pressure sensing tube. These are DIY repairs. Blower motor replacement is also within reach for a handy homeowner with basic tools. Air pressure switch replacement is straightforward. Gas or combustion work should be referred to a licensed technician.
Why does my Hayward heater show AO at the start of pool season?
AO at the start of season almost always means a bird or rodent built a nest in the exhaust flue during the off-season. This is extremely common. Always inspect the flue outlet before the first startup each year.
How do I test the Hayward combustion air pressure switch?
With the blower running at full speed, place a multimeter in continuity mode across the pressure switch terminals. A functioning switch will show continuity (closed) when the blower reaches operating speed. No continuity despite a running blower means the switch has failed or the sensing tube is broken.
What happens if I keep resetting Hayward AO without fixing it?
The AO lockout prevents ignition entirely, so forcing repeated resets does not create a combustion hazard. However, repeated blower startups with a failing motor can accelerate bearing wear. The underlying cause - vent or blower - should be fixed before the start of each heating season.

Related Hayward Poolheater Error Codes

Code: HUBHayward Pool Heater Error Codes GuideCode: BOHayward Pool Heater BO Error CodeCode: IFHayward Pool Heater IF Error CodeCode: RESETHow to Reset a Hayward Pool Heater

Same Fix on Other Brands

Pentair poolheaterERR AFS: Pentair Pool Heater ERR AFS Error

Models Known to Experience AO Errors

This repair applies to most Hayward poolheaters with this error code. Common model numbers include:

H400FDN, H250FDN, H150FDN, H400FDP

Last verified for technical accuracy on March 14, 2026