The customer stated no heat on the second floor. Turned the system on and found that it started no problem. I did see a little flame rollout upon start up. The system ran for 4 minutes and shut down for a flame sensor error. When I pulled the flame sensor out it was black, opened the burner and found that the two top burners were black and Sooted. The heat exchanger is plugged on the two passage ways of this furnace. Left the system off while I was here. Explained to the customer the issue and made him aware he needs a new furnace (LP) gas. The customer is going to speak with a home warranty company first and then give us a call for an estimate. He does not want to set up an estimate right now until he calls the home warranty company.