Hayward Heater H400FDN Error code AO

I just installed a brand new Hayward heater, H400FDN, and I'm getting error code AO when I try to fire it up. This is explained as a problem with the blower or blower vacuum switch. The blower seems to run fine while this is happening, and the tubing between the vacuum switch and the blower is not clogged. So the apparent culprit is a bad vacuum switch, but before I go through the hassle and delay of a warranty claim, anyone have any other ideas to try?
Thanks,
Andrew