The readers digest version is that apparently no one you’ve had repaired the vehicle has taken the time to properly diagnose the issue(s).
It seems they are firing the parts cannon according to what the trouble codes shows and not looking further.
FYI ….
If You have A7EB code on the dash of Your GMC vehicle that means that there is a problem that was detected with the diagnostic control module on the vehicle. This is not the same as a standard check engine light that will let You know there is a problem with the engine. Typically this code will happen when there is a fault at some point in the startup procedure of the vehicle. Usually restarting the car is enough to get rid of the code. It can also be turned on just because a sensor sent one signal that was out of range, but not enough to trip an actual check engine light.