Furnace is labeled RPJ II Comfortmaker, GUI series, maker is Inter-City Products.
It has "Total Furnace Control" electronic controller, model 50A50-110 (controller is also labeled "The General 90".
(Also, it's 15 years old - I forgot to enter the age correctly before submitting.)
Intermittently, I'll notice that the furnace blower (and the draft-inducer blower on the flue) are running, and not shutting off, even though the gas is off, and there is no heat. Plus, the fault-indicator LED on the controller is doing 1-blink, which means "system lockout".
My manual indicates that system lockout occurs if either 1) there are three failed attempts at ignition, OR 2) the flame is established but then lost after being sensed, and this sequence happens 5 times.
Unfortunately, the problem is intermittent, so I've never seen the sequence of events - just that a lockout has happened. Once the system is locked-out, it stays that way, with both blowers running, until the heat signal from the thermostat is removed for at least a few seconds. But then it seems to work fine for a while.
Things I've checked, or tried to check:
1. The flame sensor seems OK. I have the same furnace model upstairs, and measured voltages on it's flame sensor when cold, and when hot. I see the same voltages on the furnace with the problem. As a precaution, I cleaned the flame sensor by light "sanding" with a emery board (I've had a flame sensor problem in the past in the upstairs furnace, with more typical "flame comes on and then shuts right down within a few seconds". But this new problem seems different.)
2. High limit switch always seems to be closed - at least I've seen no evidence of it opening. (Plus, the controller has error codes for an open high-limit switch, and I don't see that code.)
3. I suspected a possible pressure switch problem, and maybe this is still a possibility. But at least most of the time the pressure switch seems to be doing the right thing - being on when the draft-inducer blower is running. But, maybe there is something intermittent about it. (On the other hand, the controller also has error codes for the pressure switch being stuck open, and for stuck closed, and I'm not seeing those codes.)
4. I'm not sure what the "roll-out switch" is actually supposed to do, but see no indication of an issue, based on seeing voltages on both its leads, that "seem normal". (Plus, I'm not seeing its error code, either.)
I have seen two incidents of seemingly abnormal behavior so far:
1. One time, I saw the igniter glow, and then turn itself off when no flame occurred. I was too slow to get my voltmeter on the gas valve control lead, to see if it had 24Vac applied. After some number of seconds, the igniter came on again, and this time things lit as expected (and stayed lit). Note that in this one incident, the flame never came on at all, during the failed attempt (as opposed to briefly coming on and then going out).
2. On one other occasion, I witnessed the draft-inducer blower come on, and then turn off after just a few seconds. This repeated after some number of seconds (maybe 30 seconds later). On the third try, the draft-inducer blower came on and stayed on, as it is supposed to do, prior to the igniter being energized. While this was happening, the error code I was getting was 6 flashes, which is not described in the manual (it has codes for 1 steady light, 1-5 flashes, or for continuous flashing....). (As an aside, 5-flashes means open roll-out switch.)
The manual implies that when given the command for heat, the controller first checks for a closed high limit switch plus an open pressure switch, then turns on the draft-inducer blower. After 30 seconds, it will start heating up the igniter. But before it heats the igniter, it checks that the pressure switch is closed (meaning that the draft-inducer blower is indeed working). The implication is that this pressure switch check takes place just before the igniter is energized (i.e. 30 seconds after the draft-inducer blower starts). But maybe it is checking after just 4 or 5 seconds? Could an intermittent prssure switch acount for both incidents 1 and 2?
Or am I getting flakiness from the controller itself?
The indicator of the controller thinking it has an internal problem is a steady-lit LED - I've never seen that symptom. But, I don't know how effective the controller would be at self-diagnosing some intermittent behavior.
Any thoughts? (And sorry about the length, but I wanted to be as detailed as possible!)