What will cause a 2009 Pontiac g8 gt 6.0 motor to not fire one side of motor

It's a matter of checking backwards one item at a time. If the spark plugs and wiring connections are sound you need to check the coils for those 4 cylinders on the bank that is not getting power to the plugs. Coils do fail and need replacing eventually but it could be an issue beyond the coils. Since the problem only affects one bank anything that covers both banks of the V8 will obviously not be at fault if you have one bank firing normally.

I would also get a scan tool on the car to see what fault codes it is generating.

I am reading something all about Limp Mode
or LIMP SAFE MODE...

Which seems to be all about
Thread G8 GT Limp Mode P1516 P2101 P2176

the car being driven at extremes... in other than
prescribed parameters ... causing OBD codes to be posted

My guess you are experiencing Limp Mode

I tried to pull up schematics from Autozone... but your car is not supported Looking for common coil (or coils) fuse(s).

G8 GT Limp Mode - P1516, P2101, P2176
Particularly this last thread [4 cylinder failure]
... but read on

and disconnecting the battery [ ill advised ] to reset the "condition"


Lifter failure or stuck in limp mode or WHAT is going on PLZ HELP LS1TECH

Not Limp Mode