Trane XE 78

As suggested, am reposting in a new thread.
This furnace will kick on OK at initial call for heat.....igniter will glow and flames will kick on, and unit will run until thermostat tells it that we are warm enough. Burners will then shut off, awaiting next call for heat. So far so good
HOWEVER.......next call for heat cycle, the ignitor will glow, flames will come on, but will go directly out. Couple seconds later, ignitor will glow again and flames will come up.....maybe they will stay on, and maybe they wont. IF furnace cycles thru a second call for heat, once t-stat is satisfied, blower will not go off and cold air blows continually.
If I shut the breaker off then back on, or turn t-stat off then right back on, furnace will satisfy this new "initial" call for heat, only to repeat the flame on/right back off or cycle thru a call for heat and then blow cold air after burners shut off.
Economy being what it is...any help or suggestions that might save me a service call would be greatly appreciated....
Thanks in advance....

 

See if your furnace has the typical flame sensor rod that has one skinny wire leading into the burner area. Usually 1 screw holds it in. Usually located at opposite burner from ignitor. You can remove it and polish that rod to get off oxidation. Steel wool, scotchbrite pad or real fine sandpaper works. Very very typical problem that is probably the first thing to check witrh such furnace failures as yours.

 

Since the main burner flames are reliably turning on, but not staying on reliably, I agree with Ecman that cleaning the flame sensor is a reasonable guess as the problem.

If that doesn't solve the problem, we've got lots of other things for you to check!

 

Will burnish off flame sensor......seems we replaced that a year or 2 ago......
I can understand a bad sensor NOT keeping flames lit.....but the continual blower pushing cold air escapes me.....but will take baby-steps first.

(If y'all hear a loud BOOM from the Indianapolis area, that will mean I goofed!!)