I have an old Day and Night (over 16 y/o) gas forced air heater that is blowing cool air only. The Model # is 8OU-19A or 8QU-19A. The igniter/pilot appears to be working fine & when the temp controls are raised the fan comes on but the main burners do not. Where do I start, even if the pilot seems to be working could it be bad? Does the gas valve normally go bad if so is there a test to do? Any help would be welcomed

If it has an auto pilot I would start there. A pilot type assembly that has 2 or 3 wires coming out the bottom of the auto pilot