Lenovo Is6xm Rev 1.0 Motherboard Manual -

This is the #1 power-on failure. The manual identifies: faulty 12V CPU power connector (4-pin ATX12V) or shorted motherboard mounting screw. Remove board from case, test on cardboard.

(Looking at the board with the pins facing you, typically the missing pin is top-right or bottom-left depending on the exact generation, but usually bottom-right pin 8 is missing). Lenovo Is6xm Rev 1.0 Motherboard Manual