Since the introduction of the LMM we have been getting many questions about the so called 'Programming History Tracking' and CVN's.
First, what is a CVN?, it's a 'Calibration Verification Number' which is really a fancy GM name for a type of checksum number used to validate a stock or modified tune.
To understand what this is all about you need to understand a little about checksums. A checksum in it's simplest form is a calculation of a range of numbers that must equal a known 'checksum' value.
For example assume our checksum = 10, if we add 2+2+2+2+2 the value equals 10, therefore our checksum matches. If we added 2+2+2+2+3 we get 11, therefore this would be a mismatch of the expected checksum of 10.
GM knows the expected checksums (or CVN's) of the various ECM segments, Fuel, Speedo, Engine etc. So when the ECM adds all the calibration data within a segment it must match a predefined checksum (CVN) value, if they don't match then either the calibration data has changed or there is an error with the flash memory.
If you go to this website - http://tis2web.service.gm.com/tis2web
You can enter in your VIN, choose the ECM as the module and you can then see the expected CVN's for each calibration segment.
This is what your local dealer will look for, the TechII is able to retrieve the checksums (CVN's) of each segment on the LMM, as far as your dealer is concerned these numbers must match what CalID shows, if they don't then the calibration data is not correct (not stock).
Below is a screen shot showing typical stock CVN's, these CVN's will match what CalID shows for these part numbers -
The next screen shot is what TechII will show on a tuned ECM -
You can see in the second screen shot the part number 12628693 shows 9 different CVN's, this clearly shows this ECM has at some stage had many non stock tunes put in it.
The main problem with CVN's is they aren't a simple 2+2+2+2+2 = 10 style checksum as was shown above, they are what is called a CRC which can't be fooled. See here for more details - http://en.wikipedia.org/wiki/Cyclic_redundancy_check
Some of the common questions we are asked are -
Q - Can GM really detect if my LMM ECM has ever had a non stock tune even if I've put my stock tune back in?
A - Yes as the ECM will log the CVN's of the calibration segments each time it is reprogrammed and store them (up to 10).
Q - Can I just reflash the stock tune back in 10 times?
A - No, the ECM is too smart, it only updates it's internal list if the CVN is different to one already stored.
Q - Can the programming history be erased with TechII if I slip my mechanic a $100 bill?
A - No, the ECM does not allow the history to be erased for obvious reasons.
Q - Can the CVN's be fooled on a non stock tune?
A - No, CRC type checksums cannot be fooled within the constraints of the flash memory space / layout of the ECM.
Currently the LMM is the only vehicle this programming history is activated on, but it would be a safe bet this will expand in to all new models over the coming years.
This topic was previously talked about here -
http://forum.efilive.com/showthread.php?t=7461
Cheers,
Ross