Gas furnace sometimes fails to ignite

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Old 11-16-17, 04:10 PM
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Gas furnace sometimes fails to ignite

I have a 20+ year old Trane XL 80 gas furnace. Sometimes, mainly after being off for several hours, it fails to ignite. The thermostat is set to start about one hour before I come home. When I get home sometimes I hear a small fan on, but the house is cold. If I turn the thermostat off and then on again, the heater usually ignites and after about 1 minute (when the air is warm) the big fan starts as it is supposed to.
Can anyone help troubleshooting this problem?
Thanks
 

Last edited by sssanti; 11-16-17 at 04:19 PM. Reason: typo
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Old 11-16-17, 05:24 PM
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Look thru the sight glass in the blower door. You should see an LED flashing a fault code. The descriptions are on the back of the blower door.
 
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Old 11-16-17, 06:11 PM
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Unhappy

The furnace is old, from 1986.
It does not have LED codes . See pictures:

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Old 11-16-17, 06:42 PM
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20+..... very + ...... over 30 years old.

Post the model number off the ID tag inside there by the burner.
The control box for the furnace is behind the lower door.

Have you had it professionally cleaned and serviced ?
 
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Old 11-16-17, 07:20 PM
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If you have not cleaned the flame sensor in the past year or two, it could be the problem. On your furnace it is between the number one and number two burner - counting from the left. Its held by one small hex head screw. remove it and wipe it lengthwise with fine steel wool or a dry SOS pad.
Then wipe it with a dry, clean, rag. Reinstall and test.
 
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Old 11-16-17, 08:01 PM
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Yes. I did not realize it is 31 years old. It was professionally serviced about a year and a half ago.
The model number is BLU100J948A0

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Old 11-16-17, 08:38 PM
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Try cleaning the flame sensor rod first.
Then you can try removing the rubber hose from the draft inducer to the pressure switch. Good chance it's plugged at the blower end. There is a little tiny hole inside that hose fitting that needs to be cleaned with a small paper clip, straight pin or small drill bit.
 
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Old 11-17-17, 04:38 AM
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Thank you Pete and Woody. I will try your suggestions this weekend.
 
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