Odometer readings are commonly stored in non-volatile memory. The memory on GM cars was stored in the instrument cluster on many vehicles. When a portion of the cluster failed, the entire cluster was replaced. It was necessary for the dealer to program the odometer reading into the new cluster. ECMs, PCMs and TCMs have calibration data that varies based upon the sensors and loads the controller was connected to. My last TCM design was used on nearly 30 different models. The different calibrations were programmed into the modules before they were installed in the vehicles. For new vehicles, calibration was done on our assembly line. The controller part number drawing specified the firmware and calibration.
Based upon my experience with automotive, I would think they likely are steering you straight. They don’t expect end users to diagnose and replace an ECM. Anyone doing that is expected to have the required tools. I couldn’t go to a surplus yard and pull an identical module off of a Buick and put it in my Chevy because I do not have access to the proper calibration file and programming tool. Having designed the security algorithm for the module’s boot code, I know that if I tried to reprogram the memory without knowing the checksum of what was currently in there, the programming code in the BOOTROM would erase a vital part of the firmware. Then it wouldn’t even work on the Buick until totally reprogrammed. We didn’t want “tuners” to be able to reverse engineer the firmware.
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