Hayward Heater AO Code: Causes and Fixes

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An AO code on a Hayward heater is a blower vacuum switch open error and simply means that the blower prover switch did not close after the blower started running which forces the ignition trial to go into lockout. However, the blower can still continue running when the ignition trial goes into lockout.

The Hayward Heater AO Code occurs when the blower prover switch does not close after the blower starts, when the blower prover switch opens unexpectedly during the operation, or when the blower prover switch opens during the post-purge cycle (when the heater is not firing).

A blower vacuum switch is a safety device in your Hayward heater that is designed to prevent the ignition sequence from continuing without the blower having enough airflow for combustion.

This is how a blower switch works, when the blower has enough airflow, the negative pressure created within the blower housing closes the contacts on the blower vacuum switch. This sends a message to the ignition control board that it’s safe to continue the ignition process.

Below is a chart of common causes for a Hayward AO Code

A FAULTY VACUUM SWITCH WIRINGSecure Wire Harness Terminal to Spade on Vacuum Switch
BAD BLOWER WIRING OR CONNECTIONEnsure Blower Plug Is Fastened on The Control Module
A DEFECTIVE VACUUM SWITCHMeasure Resistance on Blower Windings
BAD BLOWER RELAYMeasure If 120VAC Is Present on Blower Relay Endings
VACUUM SWITCH IS DEFECTIVEReplace A Defective Vacuum Switch

Diagnosing a faulty vacuum switch

To diagnose a faulty Vacuum switch, follow these steps

1.       The first thing you want to do is check vacuum tubing for any faults. A faulty vacuum tubing is one of the commonest causes of the AO Code.

2.       Another common culprit is a faulty vacuum switch wiring. Check for faults or breakages in the blower wiring connection. Sometimes rodents might eat up your wiring.

3.       Check if there are any faults in your blower wiring or connection. If the blower won’t start due to faulty wiring, your heater won’t ignite which could result in an AO code displaying.

4.       Take a closer look at your vacuum switch, if possible, remove it and inspect it. Check if it’s defective.

5.       Head over to the blower relay. Check whether it is defective or not.

6.       Check if you have a defective vacuum switch as a whole.

These listed parts of the blower switch are the commonest cause for your AO code on your Hayward heater. Once these are addressed, you should expect the code to go away and your heater running in no time.


For each of the above issues that cause the AO error code, the following are the fixes to each of them.

1.       Check the vacuum switch tubing and if it’s defective, replace it with a working one (preferably a new one.) If the tubing is working just fine, go the next step.

2.       If your vacuum switch doesn’t have problems, then check the vacuum switch wiring. Are the terminals intact? Are there any breakages or disconnections? Make sure to check whether the wire harness terminals are not loose. If possible secure them by fastening them to spade terminals on the vacuum switch.

3.       To check for faulty wiring on the blower, manually inspect the wiring on it. Sometimes, a loose blower plug can cause an AO error. Make sure that the plug on the blower is fastened to the control module. Otherwise, if things look OK at this step, let’s proceed to the next step.

4.       At this stage, if you have tried all the above steps and the error doesn’t go away, you might be dealing with a defective vacuum switch. Now to check whether your vacuum switch is defective or not, follow these steps:

1)      Disconnect the blower plug from the module

2)      Now measure the resistance across the blower winding

3)      Winding resistance across lead should be in the range; Red-to-White: 4 to 5 ohms.

4)      However, if the measured values vary significantly from the above values, chances are, your blower is defective. Otherwise, if they are within these ranges, then proceed to the next step.

5.       If the blower is working fine, you’ll have to check whether your blower relay is working fine or not. Follow these steps below:

1)      Start by disconnecting the blower plug from the control module

2)      Switch the heater mode to the pool or spa.

3)      Lower the setpoint temperature to generate calls for heat

4)      Now, during the pre-purge period, measure for 120VAC across pins 1 and 2

5)      If the 120VAC is not there, it’s likely that you are dealing with a defective control module relay. The solution at this point is to replace it.

6.       After trying out all steps above and the error is still there, then it’s clear that your vacuum switch is defective and needs a replacement. To replace your vacuum switch on a Hayward heater, start by removing the blower vacuum switch itself. Follow these steps

1)      Firstly, turn off everything (pump, gas supply, and the heater power)

2)      Then remove the front access panel

3)      Disconnect the wires from the vacuum switch

4)      Then pull the tubing from the hose barb on the switch

5)      At this stage, remove the screws that attach the pressure switch to the control panel.

6)      To install the blower vacuum switch, simply reverse the above steps.

Also see Hayward Heater LO Code troubleshooting.

Final Thought

When you are faced with a Hayward AO code, the above-mentioned issues are the commonest cause of the problem. Going through these troubleshooting steps should solve the problem. However, in order to keep your pool heater working in great condition, you need to observe regular maintenance. At least twice a year.

Having said that, if you are not familiar with the maintenance and repair of your heater, it’s best to call in a professional to come and give a hand. We hope you have resolved this problem though and thanks for reading.

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