Mac Pro 4.1 (I think) won't boot

My Mac Pro 4.1 (I think. I bought it at a garage sale) won't boot at all I've searched the iFixit forums and Reddit forums and all the solutions won't work I have a 5v stby dieg led and when I plug it in it flashes ovtmp CPUA and CPUB red then goes away after a second, I have the power cord jammed as far as it goes and still nothing it all started happening when I upgraded the firmware to the Sierra needed firmware and installed Sierra fine and I had about 5 mins on Sierra before it shut down and never booted again GPU is a amd Radeon HD 5770 Mac model that came with the unit I don't mind if I don't get to fix it as I only spent $20AUD on it and am baffled why I cannot figure out why it doesn't boot