I have a 12 year old Trane XE 90 gas furnace. Occasionally ( once or twice a week) it will short cycle. When gas is called for, vent fan starts, igniter glows, gas flows, but sometimes only the first 5 of 6 inshot burners ignite. The 6th, which contains the flame sensor, does not always ignite causing the gas to shut off and the unit to recyle. Sometimes the 6th burner will ingite on the 2nd or 3rd try. If not the system goes into lockout (2 flashes)