I have a 1997 vintage Trane XE80, which would cycle on for heat, Gas burners will light but for only a second or two then go off. The unit has a White Rogers Model 50A50-471 controller, The controller is flashing error code 5: Flame Sensed with Gas Valve Off. After reviewing post on this forum I remove and cleaned the flame sensor, no change. I then changed out the flame sensor with one from a second furnace. Same results.
Its not the flame sensor. A dirty flame sensor won't give you a Flame detected with gas valve closed error. Since if its dirty, it generally can't detect a flame.
Either a reversed electrical polarity, poor ground, or the board is bad. Those would be the common problems for that error code when there is no flame.
The controller board is held in with a few screws, in my case (carrier) one of those screws grounds the board to the frame of the furnace. Long use and the screw can back out a little, connection can corrode, No Ground. I would say loosen All screws a turn or two, spray with contact cleaner, and re tighten screws. By all screws I mean the thermostat leads too. Pull off the spades, spray, emory, spray, put back.
Thats a starting point, and a bad ground can really cause problems.
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