Hello, I'm new to this forum, been a Mechanic for a long time. I need some input on this one. 2009 Enclave, 163000, vehicle is in great condition, driving on thursday night - No/low power miss firing then ran normal and great then no power serious misfires. when I could finally shut down in a safe area, found front 3 cyl manifold HOT, melted sticker off of oil filter and the big lead to Alt. output shows char and blisters. Replaced both upstream O2 sensors - same - dropped front cat (melted internally) ran open header still no help .... Anyone else had this issue? Doing compression checks in am and maybe pulling intake and exhaust manifold to check valves visually. ANY help greatly appreciated, It's the wife's car.