Working on a Bryant 80%, model #: 310AAV03607AAJA manufactured in '02.
Had a client call in, saying every other day or so the unit locks out and must reset power to control. Unit is running when I arrived, and the only issue I saw of ANY question was slightly low manifold pressure of 3.35" and a semi-dirty flame sensor. After cycling it numerous times hoping to see some action, it finally showed the phantom and only 1/3 burners ignited, being the burner nearest to the HSI. I still had my manometer in the manifold pressure tap, and when this occurred there was no change in pressure. 4 trials and then lockout. It did this once more on the next cycle after reset, 4 trials then lockout. I dismantled the manifold, removed and check orifices as well as the manifold and it was all clean. Removed the burners (uni-burner style assembly) and checked cleanliness. Clean as a whistle. Reassembled and tested again, fired normally the first time, then same problem repeated the next cycle after reset. Cycled another time without adjustment and fired normally.
I then adjusted the manifold pressure to 3.5", and the last few times I cycled the unit it fired normally. The call was already a late one, and by now it was nearing 8pm and I was an hour from home, and the unit wasn't locking out at this point. The client was okay with resetting if need be to see if the problem returned this weekend. So I parted ways.
I really didn't think that a small difference in manifold pressure could create this issue, and with an intermittent problem as such, just because the client has not called back in I'm not 100% confident that was the issue.
Being that I was getting basically-normal and consistent manifold pressure through the pressure tap, while on site I'd ruled out a glob of Rectorseal from the field connection side of the gas valve being the culprit. Please correct me if I'm wrong.... as when I was driving away I was somewhat kicking myself in the ass for assuming and not ruling out that possibility.
I've seen this happen 2 other times in my career, once being a failing modulating gas valve and the other being a dirty burner assembly. And we are mostly a heat pump market in my area for residential, so maybe this happens more often than I am aware of.
Obviously 2-stage and modulating valves fire at a reduced manifold pressure far below 3.35"... so I'm not convinced this was the problem, aside from the assumption those units are designed to fire at a reduced pressure, and this 80% furnace is not. Not to mention I've seen 80% units fire just fine at a slightly reduced pressure before adjustments on PMs.
I'm at a loss at the moment, with somewhat hurt pride as we all sometimes experience, just waiting for the fella to call back in.
Any thoughts/theories/comments welcome! Thanks.