Checksum Question

[67firebird455]

First of all, thanks to the team for all of the effort put into this over the years, it looks it's come a heck of a long ways. Well done.

I've been working on my son's 2002 Trans Am, and figured I'd check out the project some more. Since we built an LS6 for it, I wanted to paste the low and high octane timing tables to it, using the fix checksum feature to show the original "1735" CVN. Am I misunderstanding what the patcher's checksum feature is for, or am I doing something wrong and should be able to manually (or editing an xml) define the checksum/CVN? I attached a couple of images to show what I'm seeing on my end.

Thanks again for any insight!

[antus]

I am not certain, but I do not believe P01/P59 have a CVN, even though there appears to be some calculations present for it in universal patcher. I think they might just do the calcs, even though the cvn doesnt exist.

[67firebird455]

I am not certain, but I do not believe P01/P59 have a CVN, even though there appears to be some calculations present for it in universal patcher. I think they might just do the calcs, even though the cvn doesnt exist.

@antus - Thanks for taking the time to respond. I verified with tis2web for the factory OS in this thing that it lists all segment part numbers and the appropriate cvn/checksum for each except for Operating System. California emissions is now checking for "modified software" now by referencing the segment part number and factory CVN/checksum. I believe I may just be confused as to how the checksum fix really works, and can't seem to find an explanation when searching here.

[Gatecrasher]

CVN is the value reported to scan tools to determine if the software has been altered. It's calculated on the fly and isn't stored in the ECU.

The flash checksums are used internally by the ECU to determine if the data was flashed correctly, and if it's become corrupted over time.

The different editing tools fix the flash checksums, but they don't do anything with the CVN. You'd have to have to rewrite the code for the OBD Mode 9 PIDs to provide a fake CVN value to accomplish what you're talking about. I doubt anyone here will help you with that. At least nobody in the legal reach of California and/or the US EPA.

[67firebird455]

CVN is the value reported to scan tools to determine if the software has been altered. It's calculated on the fly and isn't stored in the ECU.

The flash checksums are used internally by the ECU to determine if the data was flashed correctly, and if it's become corrupted over time.

The different editing tools fix the flash checksums, but they don't do anything with the CVN. You'd have to have to rewrite the code for the OBD Mode 9 PIDs to provide a fake CVN value to accomplish what you're talking about. I doubt anyone here will help you with that. At least nobody in the legal reach of California and/or the US EPA.

Thanks Gatecrasher. It's strange, because in HPTuners, the correct factory CVN (calibration verification number) = the checksum I'm seeing with a factory image. The second you make any change, the checksum/CVN updates. Cal ID = module part number, CVN = checksum2 it looks like from looking it all over in universal patcher. Is the patcher auto/checksum fix only used for data integrity?

[antus]

He is saying checksum is stored in the bin, cvn is calculated live only and not stored. So it needs a custom operating system software change, which is much more than a simple value change, to fake it.

[Gatecrasher]

I don't know enough about the internal logic of these older ECUs to explain exactly how all the code works. The CVN and flash checksums may be calculated using the same method. The CVN may just be the lower two bytes of one of the flash checksums. There's a few different ways you can do it.

When I was screwing around with my 2017 E92 ECM, there was a reference CVN I had to change to avoid triggering flash memory failure DTCs. But that reference CVN was only used to tell the operating system what the expected result would be when it ran its realtime CVN calculation. If the realtime result didn't match the reference result, the OS set the DTCs and soft bricked the ECU. There were several other flash checksums involved as well.

Ultimately, the CVN function that talks to the OBD tools is designed to prevent exactly what you're trying to do.

The only way around it is to patch the OBD mode 9 code in the OS to return a fake result, while also not breaking the flash memory integrity checks. It's not an easy job, and there's not a lot of people who'd be willing to risk the wrath of CARB and EPA over that.

[Tazzi]

For the VPW based ECUs (P01/P59 ect), the CVN is calculated by the engine computer when requested by a scantool.
The CVN is not actually written inside the bin.

Whereas, other newer ECUs have the CVN written in them, but, I can say from first hand experience, that most ECUs will disable starting and set checksum/memory related fault codes when the CVNs are not correct when written to flash.

[67firebird455]

Thanks guys, I appreciate the help.