DTC 652-6 - GM Gas Injector 2 Drive Circuit Shorted To Ground
DTC 654-6 - GM Gas Injector 4 Drive Circuit Shorted To Ground
there is no 656-6 DTC in the list

check the injectors make sure there are no shorted circuits
check wiring, plugs
- remove the coresponding plug on the injector in question, then remove the plug on the ecu with the same coresponding wires, ohm out the respective wires on ohm scale, if resistance is less than 0.5ohms then repair the shorted wires.
- if no short found in wiring then check the coils in question
using same ohm scale measure between pins on injector coil, if reading is less than 2ohms 0.5 then coil is bad, replace it.
- if reading is ok then ecu is bad.

during testing the wires and coils use these wire numbers
WIRE # 857 (WHITE) = INJECTOR 2 CONTROL DRIVER
WIRE # 861 (WHITE) = INJECTOR 4 CONTROL DRIVER
wire 751 being the common wire in all injectors

hope this helps