DIY Error Codes

Hayward Universal H-Series gas pool heater Error AO

Blower vacuum switch open. It resets as soon as the blower switch closes.

Note the code. It resets as soon as the blower switch closes. If AO comes back, call a qualified pool heater technician.

Start the fix →When to call a pro ↓
Full answer & models

Hayward AO on a Universal H-Series heater means blower vacuum switch open. If it keeps returning, Hayward says to contact a qualified service company.

Applies to: H150FDH200FDH250FDH300FDH350FDH400FDH500FD
Also searched as: Hayward heater error AO, Hayward H-Series AO code, Hayward heater A0

Gas appliance. Service only by a qualified technician; power off before any service.
Why
Hayward, what to do if you smell gas: do not try to light any appliance; do not touch any electrical switch and do not use any phone in your building; immediately call your gas supplier from a neighbor's phone and follow their instructions; if you cannot reach your gas supplier, call the fire department. Installation and service must be performed by a qualified installer, service agency, or the gas supplier. Hayward: before installing or servicing this electrical equipment, turn the power supply off. Hazardous voltage can shock, burn, cause death or serious property damage.

What AO means

Blower vacuum switch open. It resets as soon as the blower switch closes.

More detail
Hayward's Universal H-Series owner's manual (Figure 4, Error Codes) lists AO as "Blower vacuum switch open error": If the blower prover switch does not close after the blower starts (or opens unexpectedly during operation or post-purge), the control stops the ignition trial and goes into lockout with the blower running. Automatic reset is immediate after the switch closes. The manual's instruction for error codes is to contact a qualified service company for assistance.

5-minute checks

Fix it step by step

Step 1 of 2

  1. Note the code and wait

    • 2 min
    • None
    • DIY

    Safety: Gas appliance. Service only by a qualified technician; power off before any service.

    Reset is immediate once the blower vacuum (prover) switch closes.

    More detail

    Note the code and wait.

    Write down the code and when it appears. Per Hayward's Figure 4: Reset is immediate once the blower vacuum (prover) switch closes.

    Did the code clear?

  2. Call a qualified service company

    • Decision
    • None
    • Pro decision

    Stop here and book a qualified technician.

    More detail

    Call a qualified service company.

    Hayward's owner's manual says to contact a qualified service company for assistance with error codes. Installation and service must be done by a qualified installer, service agency, or the gas supplier. Do not open the heater cabinet yourself.

    Still showing AO?Need a pro → call a pro

Code cleared. Nice work.

Watch the display for a day to make sure it stays clear.

Back to top

Fixed after any step? Watch the display for a day to make sure it stays clear. Still AO after step 2? call a pro.

Call a pro if…

  • You smell gas (leave first, then call your gas supplier from a neighbor's phone).
  • AO keeps coming back or the heater stays in lockout.
  • Anything that needs the heater cabinet opened or wiring checked.

FAQ

What does Hayward heater code AO mean?

"Blower vacuum switch open error", per Figure 4 of the Universal H-Series owner's manual. If the blower prover switch does not close after the blower starts (or opens unexpectedly during operation or post-purge), the control stops the ignition trial and goes into lockout with the blower running. Automatic reset is immediate after the switch closes.

What does Hayward say to do about AO?

Contact a qualified service company for assistance, per the manual's error-code section.

Source: Hayward Universal H-Series Owner's Manual (H150FD-H500FD), Figure 4 Error Codes, pp. 13-14; safety, pp. 1 and 3. Written by the DIY Error Codes team. Updated 2026-10-16. DIY Error Codes is independent and not affiliated with or endorsed by Hayward.