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.
Nine times out of ten when I get called out for an AO code, it's a bird nest or a bunch of spider webs in the flue. If you ignore this code and try to force the heater on, you're looking at carbon monoxide backing up into your equipment area. Don't mess around with it. Clear the vent, test the blower, and if those check out, swap the pressure switch. Usually a pretty quick fix.
When your Hayward calls for heat, the control board expects the combustion blower to spin up and the air pressure switch to snap shut in just a few seconds. If that switch stays open, the board kills the startup sequence right there before any gas ever flows. I always start at the exhaust louvers first because honestly, clearing a bird nest costs you nothing and fixes 40% of these calls. Parts-wise you're usually looking at $15-80 depending on what failed.
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 locked on the display right at startup before the ignition sequence even tries to fire
You hear the blower spin up for 3-5 seconds, then everything cuts out and the code appears
Completely silent when you call for heat. Nothing happens at all, not even the blower.
Pool temp keeps dropping even though the heater display looks like it's trying to run
Code shows up every single spring startup after the heater sat dormant through winter
Can you reset a Hayward poolheater to clear the AO code?
You can't just clear this code from the keypad if the underlying problem is still there. Fix the physical issue first, then power cycle the heater at the breaker. Leave it off for 30 seconds, flip it back on, and call for heat. Watch and listen for the blower to ramp up to full speed before the ignition sequence tries. If the blower hums but doesn't actually spin up, the reset won't stick and AO comes right back.
Tools Required for Diagnosis
Phillips #2 screwdriverFlathead screwdriverDigital multimeterBright LED flashlightNeedle-nose pliersWork gloves
Diagnostic Checklist
Follow these steps in order. We start with the easiest external fixes before opening up the machine.
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
OEM Number
Estimated Price
Combustion blower motorCHXBLW001 · $100–$220
CHXBLW001
$100 – $220
Air pressure switchCHXPRS002 · $20–$45
CHXPRS002
$20 – $45
Frequently Asked Questions
What does AO mean on a Hayward pool heater?
AO is the heater's way of saying it can't breathe. Before the gas valve ever opens, the control board needs proof that the blower is actually pushing enough air through the system to safely vent combustion gases. The air pressure switch is what provides that proof. If the switch doesn't snap shut to confirm airflow, the whole startup sequence dies right there. It's a really common fault on Hayward Universal H-Series heaters, especially units that sat uncovered all winter.
Can I fix Hayward AO myself?
Yeah, most of the time. Clearing a bird nest or a blocked exhaust louver is a ten minute job with a flashlight and a pair of gloves. Replacing the rubber sensing tube is like a five minute fix and costs a couple bucks. Even swapping the blower motor or pressure switch is really just a few screws and wire connectors, no special skills needed. The only time I'd say call a pro is if you smell gas anywhere, or if you've ruled out everything and you suspect the control board is the culprit.
How do I test the Hayward combustion air pressure switch?
Grab your multimeter and set it to continuity or resistance. With the heater calling for heat and the blower running at full speed, probe across the two terminals on the pressure switch. You should see continuity, basically zero ohms, because the blower's airflow should be pushing that switch closed. If your meter reads OL while the fan is going full blast, the switch is dead. Just make absolutely sure the blower is actually spinning before you condemn the switch, otherwise you might replace a perfectly good part.
Why does my Hayward pool heater show AO every spring?
Because something moved in over the winter. Seriously. Birds, wasps, mice, and spiders all love a warm metal box with a convenient tube-shaped opening. If you're getting AO every single spring startup, the fix is a vent termination cap with a screen that keeps critters out, and maybe a cheap plastic cover over the whole unit during the offseason. I tell every customer to do a quick visual on the exhaust before that first spring startup and it saves a service call almost every time.
How much does it cost to fix a Hayward AO error?
Depends entirely on what failed. Clearing a nest yourself costs nothing. A replacement rubber sensing tube is basically free. A new combustion air pressure switch runs $15-30 for the part. A full blower motor assembly is the big ticket item, usually $80-150 depending on your model's BTU rating. If you're calling a tech, add $100-150 in labor on top of that. Control board replacement is worst case at $200-400 for the part, but that's pretty rare for an AO code specifically.
Is it safe to run my pool heater with an AO code?
No. Don't try to bypass it. The AO fault exists specifically to prevent the heater from firing when it can't safely exhaust combustion gases. If you somehow force it to ignite with a blocked flue, you're looking at carbon monoxide building up near your equipment area or patio. That's a serious safety issue, not a minor inconvenience. Fix the actual problem first. It's almost always something cheap and fast to address.