My Hayward H400 pool heater is flashing the AO code. The blower motor is only running at maybe half speed and then it triggers the AO code without firing. Should I replace the blower motor or the vacuum switch first?

it could be a bad blower vac switch, and you may look at the rubber tubing going into the switch to ensure it hasnt dry rotted.

bad blower motor not enough air pressure

Check the tubing going from the blower to the blower vacuum switch. If there are any cracks, missing, or cut, it will give the same code. Rodents like to chew on this tube. Also, if the blower is not coming on at all, then you will get the same code, and the problem is not with the switch, it is likely in the blower.

AO will apply to your blower, blower tubing, or blower vacuum switch. You need to check these items.

Check your wiring again on all the safety devices - Water pressure switch and both Hi- limits in the header.

Your problem then would be the pick up tubing and/or switch attached to the blower that closes once the blower is on to allow the heater to ignite. Pull the tubing and check it for dust or other debris clogging it and check inside the blower housing for the same. If you can test the switch under operation, you should see it remains open during start up which is why the burners won’t fire. If you do not find any obstruction (if you can, blow some compressed air thru too just to be sure) then you will need to replace the switch.

This error is produced when the blower prover switch does not close after the blower starts, the control will stop the ignition and go into lockout. Automatic reset is immediate after the blower prover switch closes.

If you keep having this problem, contact a reputable service company, especially if it's under warranty.

The link to the owners manual is below, it references fault codes, etc..

Hope this helps.

http://74.125.95.132/search?q=cache:4mAdY7bLfoQJ:www.haywardnet.com/products/Manuals/pdfs/Manual227.pdf+hayward+AO+error&cd=7&hl=en&ct=clnk&gl=us

My heater worked ok , btu made a clicking sound, now it shows a AO error? What can I do I dont have enough money to call a pool man again.