I have a 10 year old Carrier gas furnace that keeps shutting off every 24-48 hrs. First I thought it might be the flare sensor but then I checked the bottom door and it wasn't completely closed. I opened it and then closed it again properly and the furnace started running just fine. Then after a day or two, I noticed the same thing happening again - furnace trying to come on but burners would shut off after a few seconds. I opened and closed the door again and voila - everything was back to normal. A day or two later ... same thing happens again. I was thinking to tape that door but I know it's only a temporary fix. What's causing all this? Why is that door opening ever slightly after a day or two? What can I do to fix this permanently?
There's likely a switch that senses when the door is ajar attached somewhere near the edge of the door opening. Sometimes the switch presses against a little feltlike pad that can become depressed over time. That'll release the switch and tell the furnace that the door is open when it's not. Replacing or thickening that pad may well solve your problem. Oh yes, while you're in there: cleaning the pilot, sensor, and blowing out the burner box is always a good idea.
Interesting problem, I encountered something similar and will relate it as it might be something to look into:
The client complaint was intermittent shutdown of gas burner and pilot, requiring the client to re-light the pilot light. I discovered that, occasionally, when the thermostat called for the gas valve to open gas to the main burner, the pilot flame was not immediately lighting the main burner due to obstruction of some of the burner ports near the pilot flame. A build-up of unlit gas would cause a small explosive ignition which would extinguish the pilot flame, causing the thermocouple to cool and the valve to shut off completely (as designed, for safety). Perhaps late light-off is causing a small explosive ignition, blowing the door open slightly.
I simply cleaned out the burner ports near the pilot (I actually removed the burner and cleaned the whole thing) and the problem was solved.
By the way, the obstructed ports were due to an improperly capped vent pipe on the roof; rain water would drip down onto the burner and run off the side, leaving scale build-up as it boiled off, plugging the burner ports.