Heater not firing up

[wags22]

My Pentair 4000 Heater is turning on but doesn't heat up. I'm guessing it is probably the igniter, but I'm checking here for any tips/tricks/gotcha's that I should look for before ordering a new part. The heater was installed in 2011 and is exposed to the elements all year (I live in southern CA, about 30 min from Palm Springs) so I'm not surprised if it needs a new part.
Thanks for any help you're able to provide.
Wags.

 

[Richard320]

Look inside for spider webs. As I understand it, the blower starts and then it checks for a draft in a tube before the gas valve opens and the ignitor starts. It's quite sensitive and spider webs can restrict the flow enough to keep it from firing.