Chevy express 4500 diesel cutaway. fuel oil heater (foh) is an auxiliary heater located under the cab of the vehicle and exhausts in the same area. odor of partially burned fuel and a strong exhaust small would enter the cab of the vehicle during low speed operation in cold weather. due to the intermittent character of the odor a home co monitor was placed in the cab of the vehicle which did alarm, prompting the vehicle to be removed from service and sent for repair. the foh required replacement. *tr