Lennox G32 Recurring Failures

Gentlemen,
I have stumbled upon a Lennox high-efficiency furnace model G32V5-100-3 that has started to lock out intermittently, usually at night during the past 2 months or so. The customer says that this furnace is 3-4 years old.
The diagnostic flashing lights indicate the same problem every time: pressure switch problems. According to the chart, this could be the pressure switch opening 5 times during one heat cycle, the switch closing before the call to start the combustion venter motor, a plugged condensate drain, a blocked inlet or outlet vent pipe.
I have spoken with Lennox tech support twice and they seem adamant that the problem is a water drain problem.
I have variously:
-replaced the pressure switch -removed and inspected the venter assembly and the rectangular plastic drain trap box to which it is attached - no visible problem -inspected & flushed all the water drain tubes, etc -measured the pressure differential across the switch - OK -watched the venter motor coast for minutes after the heat cycle ends, no problem with bearings in that motor -checked the inlet & discharge vent pipes - no evident blockage -climbed into the area directly above the heat exchanger and visually scrutinized the heat-x as thoroughly as possible from that angle - nothing out-of-the-ordinary was seen
I think I have eliminated the following as possible factors in the problem:
-night set-back - it still fails with setback disabled -outdoor temperature - it has failed at various temperatures
I disconnected the inlet vent pipe from the furnace so that the unit can draw air either from the connection to the outdoors or from the building and left it for a few days like that to double check the the inlet pipe - it still locked out occasionally. I have changed the main blower from continuous to intermittent without solving the problem.
Lennox stated that these furnaces may have a vent length of "25 equivalent feet" and this one might be near or a little over that. But it has worked well for a few years. Also, the vent pipes are pitched properly.
The furnace room is unheated and was 50F on a cold day. I have wondered if this might be a problem re:possible condensation in the pressure switch but the other 5 similar furnaces in the room are fine.
I have considered swapping components between furnaces to see if the problem travels or stays. But this furnace is a G32V5-100-3 and all the others are dash-4. I do not know if this is a significant difference but some of the part numbers that are stencilled on some components are different.
I think I am near the end of the list, at least my list. Lennox didn't really have anything to add.
Can someone help?
Canucklehead Toronto snipped-for-privacy@sympatico.ca
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On Sat, 19 Feb 2005 11:07:31 -0500, Canucklehead

Gentlemen,
Thanks to the guys who took the time to respond to the question that I posted on February 19.
To Turtle: I apologize for not mentioning in the original post that the drain lines from the evaporator and the burner were already separate and neither of the was connected to the drains on the other furnaces in that room.
To Zyp: A tattletale would not work on this system. The air proving switch must prove open at the start of the heating cycle before the combustion venter motor is energized. A tattletale would either pop on the first cycle or produce a fault condition.
To Bubba and Joseph: I changed the "p-o-s" Sure-Lite board on February 25 and that was that. There hasn't been a subsequent failure. Yes, apparently I was chasing my tail. Another lesson learned.
Thanks again to all.
Canucklehead Toronto
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On Thu, 17 Mar 2005 15:22:46 -0500, Canucklehead

Thats one of those where you just have to know about Lennox's problem. The idiot light code on that one was no help (usually never is). Luckily, I found someone at Lennox in tech service that knew about the problem immediately. ............and all under warranty..............NOT! Bubba
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