Pentair MiniMax NT-400 not lighting off
I've got a Pentair Mini-Max NT 400kbtu heater from around 2002 (I know, I know, but really, it's been good) that won't light off. It gets pretty heavy (nightly) use year round since new.
Tonight, no go. The red Service light is on. I immediatelly guessed it was a bad igniter, so I removed the igniter and ohmed it out - 141 ohms. This is right within the specs I've seen online (50-300 ohms).
I put it back together, and turned it on again. The system comes on, blower runs, igniter glows, valve clicks, (smell the usual small amount of gas), but then the valve clicks OFF 2-3 seconds after opening, without it firing up. It will cycle a few more times but it doesn't light.
I am able to spark it up with a BBQ igniter while the gas is flowing, and it seems to run fine after that (but won't re-ignite on its own).
I did notice, however, that when it is firing, the "central burner pipe" (the one just under the igniter position) is NOT aflame. All the tubes on either side (maybe 4 per side?) are firing and look good.
Once firing, I AM able to light the jet firing into the open tube (just off the horizontal gas manifold). The flame burns in the "air gap" (open area just after the jet), but the main burner tube associated with that middle jet does NOT ignite.
I haven't ripped into the burner system as yet. Anyone seen this before? Am I headed for a burner assembly replacement, or something else? Is this condition repairable (i.e. cleanable), or is it a rip-n-rebuild project?