I have this issue i have a 87 camaro did a 04 5.3 swap and am runing the drive by wire and it has a code P1125 it will crank and it wants to start and then dies i have spark and fule pulse and all the wirering is good and a new TB, Tach Box and Acc pedal would i need to reprogram the ecm or a relearn of some kind?

 

Spark and Fuel, have nothing to do with DTC P1125.
The Engine is most likely stalling as the PCM/ ECM and/ or the Throttle-Body does not have "good" Accelerator Pedal Position Data.

Generally, DTC P1125 pertains to the Circuit for one of the Accelerator Pedal Position Sensors.
Are you using a GM Electronic Accelerator Pedal, and a GM Electronic Throttle-Body...
or are you using Aftermarket components.
The GM components usually have two position sensors within the Accelerator Pedal to compare Position Data.
The GM Throttle-Bodies usually are the same.

For starters, the PCM/ ECM, Throttle-Body, and Accelerator Pedal, all need to be compatible with one-another.
I would carefully examine the Wiring Between the Pedal and the Throttle-Body.

There could be a Wiring Fault, a Position Sensor Fault, or even incompatible Parts.
Were Schematics included with the Parts that you purchased?
If you have GM Parts... do you have a 8-Pin or 6-Pin Electronic Throttle-Body Electrical-Connector?

 

So the TB(8pin) and Tach box are both new GM original parts

The harness is good i checked for continuity from the TB conector to the Tack box and then from the box to the acc pedal no high risistance or shorting of wires together grounds are good and power is there i also have 2 5v ref.

The only other thing i would not bet on is a new GM original acc pedal sensor but i have 5 different ones all of the same 04 but none make a difrence.

 

Are the GM components all from the same model vehicle? Truck pedals and corvette pedals, for example, are wired differently.

 

Yea there all truck parts off a 2004 so thats what i bought as well but i talked to a old coworker that now works for chevy and he sed i may need to reprogram/relearn some things in the pcm for the new TB and Tach box so ima see if that helps and if it dose i think im goldin

 

Yes that is a definite possibility.