I perform maintenance on several carrier roof top units and have come across a recurring problem that I am having trouble pin pointing the problem. Model #48TJD014---541-- (All units have the same model) The units cycle in heat mode for a few days than stop, control board indicates ignition lockout. The inducer motor runs constantly however flame does not ignite. If I turn power off and on, heat turns on immediately (inducer, flame and blower) and it works fine for a few days. I changed all limits (2 high temps and hall effect sensor on inducer fan) and it is still locking out every few days. Three out of the seven units are doing the same thing. Please advise, and I missing something, is this a board issue - if so, why three units at once.