this code E-23 is for the throttle sensor
this sensor is inside the throttle body
what is happening is when you turn on the key the ecm automatically sends a check signal to the accelerator and throttle body circuits to check them. This is what you hear when you turn on the key, you hear the throttle body calibrating or checking itself against the accelerator sensors to make sure all is ok. If during this check mode the ecm see's any of the sensors in the throttle body or the accelerator being off it will set the code E-23 and put the truck in limp mode and the dash MIL light will turn on.

If you have already checked the wires at the throttle body connector and they are good then run some calibrations first before replacing the TB.

There is a hpk document out from mfca about this code
it says to run the following 3 tests
Accelerator Released Position Learn.
Throttle Valve Closed Position Learn.
Idle Air Volume Learn ( TAS Fast Normal )

to do the accelerator released position learn do the following:
Accelerator released position learn:
a) Turn key switch to the ON position and count out 2 seconds.
b) Turn the key switch OFF and count out 10 to 12 seconds.
c) Turn the key switch ON and count out 2 seconds.
This learn is completed. You may key off now.

to do the throttle valve closed position learn do the following:
Throttle Valve Closed Position Learn:
a) Turn the key switch ON and then OFF.
b) Count out 5 seconds after turning the key switch.
c) Listen for a movement sound at the Electronic Throttlebody. It must operate.
If so, this learn is completed.

next the Idle Air Volume Learn needs to be done (this can only be done using the diagnostic program from laptop).
Air Idle Volume Learn (Fast TAS Normal)
a) Open diagnostic program version 3.1 or higher.
b) Start the engine and let it warm up to 76 to 84 degree C.
c) In the diagnostics program select model.
Select Connection: VCM1 and then
Select Target : ECM.
Select Active Test: Page 1 will open.
This will be identified with Fast TAS Normal.
To the right of the screen push the EXECUTE button.
The ECM will perform a self learn. It should complete in 15 seconds.

If this does not fix the problem and the code returns you should replace the throttle body. DO NOT try and repair it, replace it!

good luck