I have a Laars LX-400 pool heater with an 'Air Flow SW' error message. I have replaced the blower pressure switch to no avail. Any other suggestions? When the heater is turned on, it hums as if it is trying to start, but the blower does not come on, and after a few minutes the error message pops up.

I have a converted propane to natural gas Haywood H100 pool heater. I used Haywood parts to convert. when I turn the thermostat on the unit goes thru its start up cycle. Blower comes on, gas valve opens, and igniter starts and runs for aprox 20 seconds, then shuts off. I can hear the gas valve open and I smell gas in the exhaust vent. I pulled the electrode out and turned on the unit but did not observe spark at the electrode. so I replaced the electrode at $80.00. No difference.I looked for any obvious wires disconnected. don't see anything. I assume this is a spark issue, but I am not sure where to look next and what to check. any help would be appreciated. Dave Checked spark: pulled out igniter probe. grounded to earth ground and turned on unit, ran thru its ignition cycle. I hear the igniter clicking but see no spark between the ground probe and the ignition probe. moved the probes closer together, no diffrence. what would cause this condition?