I have an ~28 yr old Carrier 58DHB095-CC natural gas heat/AC unit mounted horizontally in my attic to heat the second floor of my house. Pilot won't light. I replaced the thermostat but that didn't help. When the thermostat calls for heat, nothing happens. No clicking from the ignitor. I am guessing the problem is in the ignitor, the pilot assembly or the fusible link but I have taken them all out and they appear fine?? Anyone know what might be the culprit? I'd like some advice before I start buying parts for this old furnace.