I went on a service call today & homeowner said that furnace would come on & run for @ 30 seconds & then shut off.The furnace is going through the complete cycle 1.The IBM is coming on 2.the HSI is coming on 3. the burners are lighting & running for @ 30 seconds & then the furnace is shutting off. I checked the ground on the furnace,the high limit, the pressure switch & tubing,& all were ok. I will say that once when the IBM was cycling on is when it did this. Any help would be appreciated!

Coleman-Evcon furnace- Model#BGU 075 12A