I think I've tracked my issue down, but I thought I'd post to see if anybody has any pointers or other thoughts before I go to the trouble of tearing this unit apart.
My pool heater is one year old, and it worked fine last year. I tried to start it up after uncovering my pool recently, and it does everything normally and I can hear the flame roar to life. Then, I hear a click and the flame goes out, and it does this 2 more times before giving me the IF code. I haven't verified the gas pressure as I don't have the equipment to do so, but I don't think that's the issue as no other household appliances have given me any issue, and as stated, I get a short-lived flame. I thought I might have a blockage as others get due to a spider web or something, but I pulled the manifold and there were no signs of a web at all actually, even though the back part of my heater is a spider breeding ground apparently. I have pulled the flame sensor, examined it, cleaned it, and replaced it. Wiring all appears to be ok, and just for a little test, I unplugged the flame sensor and noticed no difference. I'm not really sure what to make of that. It could be that the flame sensor is fine, and that it's kicking off for a different reason. Or it could be that the flame sensor is bad, and whether or not it's in the system, it's not detecting a flame and shuts itself off. One reason I think the flame sensor is ok is that when I pulled the manifold to check for webs the one time, it didn't have any, and I couldn't find anything else wrong so I reassembled the unit, and tried to turn it on. It didn't work a few times, but then once it just magically worked. I just let it run for an hour or so and turned it off. It rained that night and for the next day or two, and when I came back to it--it's not working again.
So, I THINK I have water inside the unit, and that maybe there's a moisture problem in my burn tubes or whatever the technical name for them is. My gutters are clogged, and I need to get someone out to clean them (I have a tall roof and a short ladder). I'm wondering if the section over my heater isn't draining down on it pretty hard and causing water (and lots of it) to get inside of it. The problem with this scenario is that I had this thing taken apart to get to the manifold, and that wasn't terrible, but going any further seemed to be like considerably more effort because I was seriously considering it the day I took the manifold out and didn't find a spider web.
Any tips or suggestion from anybody that may have had water get inside the wrong part of their heater before?