We diagnose about a hundred or more " sooted " carrier 58 series furnaces every season. We order the primary's and the secondarys, as well as the coupling box. 90 percent of these furnaces were on lp gas. Some were serviced yearly and others were neglected, some were over vented, and excess air readings were too high, or too low. The smell is one you will never forget, which are aldehydes produced from incomplete combustion. I am happy that carrier has redesigned their heat exchanger recently.