I'm an ASE Certified Diagnostic Technician with about 20 years under my belt. I'm dealing with a similar problem at present. I believe the first step is to ensure you are correctly identifying the component your referring to, there are several components that can easily be misidentified as a "computer". Once you know the components actual name (pcm or powertrain control module is correct in your case), you will need a list of pins (or "pinouts") on the related connector(s) and their relevant values (in this case KOEO (Key On - Engine Off)). Now the priority is to utilize a DVOM (Digital Volt Ohm Meter) to verify the integrity of the power supply wires and ground wires. You can do this easiest by detaching the connector(s), and probing the correct pins for the value in volts.
note: If you must purchase one, then be sure that it has at least a 20 Mega-ohm impedance or more. Any less and you chance harming your computer if it is still functional.
Once that is done, assuming you know how to use the meter correctly, you can continue testing the sensors and actuators also connected to the remaining pins in the connector(s). If all the values are as expected according to the list you attained, it would be safe to condemn the pcm as faulty.
this procedure is required by all new/reman computer dealers for a good mechanics to have any warranty honored.