Trane XE78 home furnace behaving erratically

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Old 11-19-12, 04:05 PM
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Trane XE78 home furnace behaving erratically

My home furnace is behaving erratically and I'm not sure what to do. At times, it functions fine, turning on and running through its cycles normally. Sometimes, though, the blower will stay on much longer than the burners, will continue to blow cold air, and will shut off only when the thermostat is lowered. At other times, the heating cycle will begin with the ignitor heating up, but the burners will only ignite for seconds, shut off, and then the ignitor will try again several times with the same result. The fan will eventually begin to blow after this repeats itself several times. I'm thinking that the control board is getting wacky, but I really don't know and would like some advice before spending the money on a new one. Thanks
 
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Old 11-19-12, 04:14 PM
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Take the cover off the burner compartment of the furnace. Obtain the model of the furnace off the rating plate in the burner compartment and post it here.

Observe the sequence of events that occurs at the furnace when you turn up the thermostat and post that sequence of events in order and in detail.
 
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Old 11-19-12, 09:14 PM
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The model # is TUD080C936A. The sequence was as follows:
- the ignitor came on 12 seconds after the furnace "kicked on"
- about 10 seconds later, the burners lit
- the burners turned off about 9 seconds later
- the burners came on 18 seconds later, shut off after about 2 seconds; this cycle repeated three more times; the ignitor stayed lit throughout
- after the fourth cycle, the blower turned on and stayed on until I shut it off 4 minutes later by lowering the thermostat.
 
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Old 11-19-12, 09:23 PM
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The most likely problem is the flame sensor. Pic is below Could look like that or have a right angle bend in it. It's held in place with one screw. Disconnect the wiring carefully, remove that screw and remove flame rod. Clean the shiny part of the rod with steel wool or scotchbrite pad. When putting back into furnace - make sure there is no rust between flame sensor bracket and mounting location.

Also of concern is that the control box is properly grounded. Sometimes mounting locations get rusty causing it to lose it's needed ground. Check mounting screws and associated areas.
 
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Old 11-19-12, 10:00 PM
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Sounds like a good description of the ignition sequence
caljp. That is very helpful when trying to determine what the problem might be.

PJ Max is providing you with good ideas to pursue.
 
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Old 11-22-12, 06:36 AM
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I did what you suggested and, voila!, it worked like a charm! Thank you both very much for your help! Happy Holidays!!
 
 

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