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You may have high ground resistance in the EST circuit, or a broken/ shorted wire in the reference circuit between the module and the ECM. ALso, I don't have my PROM index handy at the moment, but something sticks in the back of my mind about some chevy truck calibrations setting false code 42's; there may be an updated calibrator for your computer to address this (check with your local GM dealer parts department; give then your current calibration number, and they can easily tell you if there is an updated calibrator available for your truck).
I would suggest taking an ohmmeter, disconnecting the module and ECM plugs, and checking for shorts to ground or high resistance on any of the wires related to ignition operation.
Here is a link to a web site that describes basically how the EST system in your truck works; it may be helpful to you: