Yeah, I'm thinking it might be getting to that point. I'm pretty good with this stuff, but don't have a ton of experience with power vent systems. Trying to learn something in the process (and I have already), but I won't do anything stupid to screw up the safety systems in place, which would potentially cause a dangerous situation.

macplumb... which line are you referring to? The one on the back bottom of the heater? I drained out that line until it no longer gurgled at one point to see if that was the problem, and it didn't do anything different as far as errors go. It definitely has some suction coming through that line when the blower is on. After that test, I put it back into a trap setup per the manufacturers instructions... it shows it having a trap there. I was curious why it keeps gurgling though, seems like if the trap was functioning correctly, it would be silent? Maybe this is a problem? Is my trap setup not holding enough water to seal it off? Instructions say that the heater will prime the trap on its own during first full heatup cycle, but I went ahead and primed it part way, which would seem to indicate that it doesn't need to be open or closed there to fire up... Any thoughts about that?

I may still take a look at testing the pressure switch (today was another long crazy day with complications and fires to put out, and I had no time to look at the heater again... maybe tomorrow or over the weekend) based on the bulletin A.O. Smith released. If its just a bad switch, I know I can do that fix easily enough. I'll see if there is a leak on the air tubing that I can detect as well, thanks for that tip Jim. I'll also check the vent terminal for blockage when I can get a ladder set up (there is too much machinery doing its work outside to get up there right now). If it ends up being more complicated than one of those things, I'll call out the pros.

Thanks all, for your help.