Performed Furnace diagnostic on 2017 Goodman heater. Upon my diagnostic customer Had stated that he has had nothing but problems with the previous company that has been doing the contracting work for his house. He was told that his heat exchanger has a crack, that why the Carbon monoxide detectors are going off. Upon my diagnostic I first found the exhaust vent had been completely pulled off along with nothing but soot coming out of the vent. The reason for the high amount soot Was caused by a failure to convert natural gas set up to propane. This has caused the propane to form into this clogging up the heat exchanger. With the condition of the heat exchanger being filled with soot this is extremely unsafe and has ran like this since 2017 without being converted. Found that there was also no emergency drain pan, they disconnected primary line coming off of the evaporator coil along with none of the duct work being sealed up on the supply take offs. System was installed poorly and unsaf