Furnace won't say on

I'm having a problem with my Coleman Evcon furnace that I'm hoping someone might recognize.
Basically the furnace turns on and operates fine, but then after a short while will turn back off again. This happens quickly enough that the house never gets a chance to warm up to the set temperature. The thermostat is still calling for heat (there's a light) but the furnace ignores this and remains off.
Resetting any part of the system makes the furnace come back on immediately. What I mean is that if you pull the batteries on the thermostat and put them back in, up it goes. It also happens when the thermostat changes programs, from night to morning for instance, which I _believe_ cycles power on the control circuit. This also works if you cycle the power on the furnace itself.
I have a _feeling_ that the furnace control board is getting into some "state" that makes it ignore the fact that the heat signal is turned on. Resetting the electronics fixes the problem. But if this is the case, wouldn't the status light be trying to tell us this? Throughout everything the service light has remained a perfectly steady green.
I just pulled the filter and it definitely is a problem. But again, if this was the problem wouldn't there be some indication? Why would the furnace simply shut down?
Maur
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It may just be a dirty flame sensor. Don't know your specific furnace but is there a light window/sensor in the path of the flame? It should have some electrical wiring running from it. Just clean the window with a Q-tip and alcohol.
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On Sat, 01 Dec 2007 06:46:37 -0800, Maury Markowitz wrote:

So was the filter the problem?
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You said: "after a short while will turn back off again"
Define a "short while". 30 seconds? 5 minutes? Time is relative.
Does the blower come on? If the filter is so dirty that it is restricting air flow, you could have an over heating problem and the furnace may be protecting itself by shutting down.
(I once worked for an IT Director of a major corporation who would pretty much bite your head off if you answered a cost question with something like "Oh, not too much" or a timeline question with something like "a long time". She would very quickly let you know that plans and decisions can't be made based on answers like "pretty soon" or "very expensive". Specifics were required. The whole project team would cringe when a newbie tossed out an answer like that. We all knew what was coming next.)
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Sorry about the tardy reply! Cleaning the filter immediately cleared up the problem.
They _really_ need to have an indication like this. It would have saved me buying the new thermostat...
Maury
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Maury Markowitz wrote:

Try using a schedule for cleaning your filter. Say, once a month.
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