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Old 10-22-2011, 12:02 PM   #1
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i have a high efficiency lennox furnace. in heat mode it comes on for about 1 min and a half to 2 mins before it shuts down. the flame sensor was replaced ysterday and the pressure switch last year. the flu pipes seem to be clear as well. when it shuts down the pressure switch is dropping out along with the flame sensor, gas valve etc.

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Old 10-22-2011, 12:28 PM   #2
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Model number? Sounds like a control board issue.

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Old 10-22-2011, 01:01 PM   #3
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control board is flashing normal operation
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Old 10-22-2011, 03:10 PM   #4
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Was this before or after you pulled the door? Go to Radio Shack and get a set of alligator jumpers that you can use to jump across the suicide switch that is normally closed, when the bottom panel is put in place. That way, the system will run and you can see the codes, when it codes out. Also, try draining the water from the box on the side, and try another thermostat. Thermostats do go bad.

Tred also as Marty asked, need a model#.
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Old 10-22-2011, 04:53 PM   #5
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If the control board is flashing normal then either the board is faulty or there is an external thermostat issue which is shutting it down. I would remove the thermostat from it's sub base and if you have only two lines, jumper them together. If it has more than two lines I'd jumper the thermostat lines where they connect to the mother board at positions R & W. If it works when the stat has been bypassed your solution is easy. If not then it's $$$ time.
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Old 10-22-2011, 08:14 PM   #6
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24 volts at the board between w and common never drops out. so the stat seems to still be callin for heat
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Old 10-22-2011, 08:28 PM   #7
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i already drained the trap and ive seen the codes flashing with the door on and off, it is flashing normal operation. i cant tell what the model # is cause its all faded. when the pressure switch, flame sensor and gas valve drop out it flashes normal operation and everything comes right back on, it fires again, runs for a few mins and then shuts down again
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Old 10-22-2011, 08:36 PM   #8
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Post a picture of the unit with both the doors on and off, and a picture of the control board. Also, any serial number on the control board helps.
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Old 10-22-2011, 09:03 PM   #9
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i would but i wont be back there till monday. could it possibly be the secondary heat exchanger maybe being blocked causing a pressure issue. but the more i think about it. i dont think the inducer draft ever shuts off during the 4 times r so it goes thru its cycle until it shuts down and the blower comes on
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Old 10-22-2011, 09:06 PM   #10
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the furnace also has a burner box that the burners r in
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Old 10-22-2011, 09:07 PM   #11
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its the elite series i believe
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Old 10-22-2011, 09:33 PM   #12
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All righty now. We know that it has a burner box, and it may or may not be a elite series.

Tred, get all of the info you can on it, and post back. If you have a droid or iPhone, download the forum app and use that.
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Old 10-23-2011, 12:45 PM   #13
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No were the pressure switch's go to the header box. Clean out the ports and call the people that install that. That header box is under recall. The ports are too small and lime up fast.
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Old 10-23-2011, 12:59 PM   #14
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actually its not that the ports are too small. its the shape behind the port. the new ones have a cone shape and don't allow condensate to sit there and deposit minerals as they evaporate.
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Old 10-23-2011, 02:19 PM   #15
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i already went thru the ports and there clear

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