Dj Pool

I'm trying to help my father in law trouble shoot his pool heater. Sorry in advance if I'm not giving enough detail as I know nothing about pools. My father in law has a Laars Lite 2 pool heater. The heater worked great last year and this year it won't heat. There is a strong flame on the pilot light, and there's good circulation in the pool. Any idea what's father in law could look at next to try to determine why the heater isn't working? He's had a pool for 20 years(heater hasn't that old) so he knows his way around some things, but he's stuck on this one Thanks!