Welcome to GMT Nation...
To address your
P0068 Trouble Code... The Most Likely Culprit due to its almost Constant Use is your
Accelerator Pedal. Unlike the Old Days where the "Gas Pedal" was an Analog System of having a Wound Wire Cable and Bell Crank assembly that Physically Opened the Throttle Bodies of Carburetors that had Coiled Springs to Close them at Idle... The Modern
ACC Pedal is just a "Package of Electrical Potentiometers" that sends
"positional" and
"rate of change" information to the PCM and to the Electronic-Motor Driven Throttle Body to adjust the "Butter-Fly Plate". The
GM ACC Pedal uses two distinct High and Low Voltage Signals along with a Class 2 Databus 5 Volt Reference Signal to and from the PCM to achieve its functions.
If it Fails... The PCM is Coded to
Automatically Drop into a Reduced Power Mode...
especially if the Vehicle is engaged to Drive in Reverse. This Safety Consideration is called
"Going Into Limp Mode" and thereby points to either the
ACC Pedal ...or the
Throttle Body as being the possible Faulty Components. There are Videos on Youtube made to show you
"How to Diagnose the ACC Pedal using a Digital Multi-Meter". I'm suggesting that you
Start by Looking for an ACC Pedal problem First...as getting a replacement is only around $40.00 and that is much less Pricey than just purchasing a New GM 4.2L Throttle Body without knowing if that component is at fault.
The
P0300 Codes can have causes ranging from a Dirty MAF/IAT to a Failed Upstream O2 Sensor to having Loose Intake Manifold Bolts and/or other Vacuum Hoses on the Engine leaking. Study this Video for the means to improve your understanding of "The Modern Drive-By-Wire System" that is at the Heart of this issue: