Unit runs on lp gas, unit is 3900ft above sea level, horizontial in basement, supply air is right side. Found 43 error code active then code 14 or 12 basically shuts down. Found abs on side of combustion box melting and charred and face plate looking in to gas burners melting too. Poor flame at top #1 burner # 2 burner blue then 10 seconds later on burn turns orange and bounces as if positive pressure is in heat exchanger. Than shuts down with error codes described above. Poor flame . Than removed combustion face plate, turned on and unit ran without shutting down, still poor flame through out except # 2 below flame sensor, was orange and flame wavering upwards as if no suction from inducer fan to help. Flamesensor picking up flame but not from its top burner barely alive.QUESTIONS: WHAT LP GAS PRESSURES SHOULD I EXPECT AT GAS VALVE AND OUTPUT OF GAS VALVE. WHAT PRESSURE SHOULD I WANT TO SEE ON 1ST STAGE, THAN 2ND STAGE. 2:WHAT SIZE ORIFICE FOR LP GAS AND ALSO ALTITUDE 3900, DOES THAT CHANGE THE SIZE? 3:HAS ANYBODY SEEN ABS MELT WITH A POOR FLAME, OR IS THIS PATH OF LEAST RESISTANCE HEATING COMBUSTION BOX AT TOP?