Here's what I read on the internet:
How does a mechanic diagnose the P0601 code?
•A mechanic will begin by hooking up a scan tool to the DLC port and checking for all codes preset including history and pending code.
•Next, freeze frame data will be noted for every code. This information is critical as it tells us what conditions, such as RPM, engine load, and coolant temperature the vehicle was under when the code was set.
•All codes will then be cleared and taken on a test drive to confirm any issues found, the test drive should be under similar circumstances to when the code was set.
•Next, a visual inspection will be performed to look for any damaged wiring or circuits.
•Then, a multimeter will be used to check for proper voltage supply going to the ECU.
•Lastly, the ECU will need to be tested. Each vehicle has its own testing procedures for the ECU.
Common mistakes when diagnosing the P0601 code
The most common mistake is not checking that the ECU is receiving proper voltage, the ECU then tests bad and is replaced when it is not the cause. All steps must be followed as P0601 can be a difficult code to diagnose with intermittent issues.
And before reading this I was going to suggest checking the connectors before automatically buying a new ECM.
Especially if the problem went away. Computer faults in the silicon seldom fix themselves but electrical connectors have intermittents all the time.
If you buy a new once you can give them the VIN and they will program it for you so it is plug and play. If you get one "used" then I think you are stuck with having a dealer reprogram it.
Where is it roughly located? Roughly somewhere toward the front of the vehicle.
Don't know and can't seem to find that info for the 5.3L. It is on top of the engine on the driver's side for the 4.2L.