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Thread: LMM Programming History & CVN's Explained

  1. #1
    Forum tyhee Site Admin GMPX's Avatar
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    Lightbulb LMM Programming History & CVN's Explained

    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
    Last edited by GMPX; July 2nd, 2008 at 12:47 AM.
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    Ross, now that you have explained it will there ever be any hope of erasing this with EFI live.
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    Forum tyhee Site Admin GMPX's Avatar
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    Sorry, I don't want to say anything with any certainty on that just yet

    The tough part is there is a number of very valid reasons you would want to erase the history, one such reason would be a used ECM pulled from a wrecked truck that had been tuned, but was to go in to a stock truck to replace a failed ECM. The stock truck in this case would have warranty voided because of what was done to the other truck in the past.
    That would be like the ECM gets a bad credit rating with one owner, the next owner of the ECM happens to be Donald Trump but the bad credit stays, in that case the bad credit should be able to be wiped.
    However, chances are 90% of people will only use it for the (IMHO) wrong reasons when disputing warranty claims.
    It's a tough call because I fully understand where GM are coming from implementing this, but, it also catches out innocent parties. But I am not here to be the morals police.

    Cheers,
    Ross
    Need official EFILive help, please go here.
    For tuning support please post your questions on this forum (or other auto forums).
    Sorry if I don't respond to your PM, don't take it personal.


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    Thans Ross, I understand were you are coming from especialy when some body blows stuff up with a big ass tune and expects GM to pat him on the back and say we gotcha covered. But when you go in for a simple ECM-BCM reflash and they give you the on any future warranty because you have a little history in the ecm that sucks.
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    you wont see any info on clearing the data on an ecm for some time. logic tells me that until the lmm is done getting updates from gm, and the first models are done with the 100000 mile warranty you see it. because if an ecm fails under warranty gm pays any way so no need for a junkyard unit.

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    what happens if you read the stock tune then take that ecm out and get a differant ecm tune on that one if there is some warranty work needed or recall then put the stock ecm back in befor you take it to the dealer ?

  7. #7
    Joe (Moderator) joecar's Avatar
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    Quote Originally Posted by 69camaro5speed View Post
    what happens if you read the stock tune then take that ecm out and get a differant ecm, tune on that one, if there is some warranty work needed or recall then put the stock ecm back in before you take it to the dealer ?
    That is the "best" way to do it.

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    Lifetime Member McRat's Avatar
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    Ross.

    I'm getting the same CVN with a HEAVILY modified tune and with a stock tune flashed with a Tech2.

    This is on a 2005 Duramax.

    Seems the Tech2 doesn't "calculate" the CVN on this model, it just reads it. If EFILive does not alter those 2 bytes, it won't change? Dunno for sure.

    If it's the ECM that is generating these CVN's and the Tech2 just reads their stored location, the ECM should have a value that tells the Tech2 where to look for these. Move the pointer?
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    Lifetime Member Cougar281's Avatar
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    IMO, the problem with the flash history isn't so much the people that blow something up with a huge tune then want GM to fix it, but the people that run a 65hp Predator tune (for example) and have the unfortunate luck of an injector hanging open and nuking a piston (again, for example). The problem is the tuner did not cause the failure, but because of the programming history showing a non-stock program, GM could/would tell you to pound sand.... I'm all for the "you play, you pay", but when something happens like an injector hanging causes a motor to fail and GM says no becasue you had *any* programmer, I don't agree with. JMO.

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    Quote Originally Posted by McRat View Post
    Ross.

    I'm getting the same CVN with a HEAVILY modified tune and with a stock tune flashed with a Tech2.

    This is on a 2005 Duramax.

    Seems the Tech2 doesn't "calculate" the CVN on this model, it just reads it. If EFILive does not alter those 2 bytes, it won't change? Dunno for sure.

    If it's the ECM that is generating these CVN's and the Tech2 just reads their stored location, the ECM should have a value that tells the Tech2 where to look for these. Move the pointer?

    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.

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