Also I was told by a well known fiesta tuner that an after market tune will change the ROM ID. I think that is how they flag tuned cars. I always felt the check sums was bs.
You may be a bit confused about what a checksum is and how it actually is used in the ECU. Checksums are a simple error correction method that at the most basic, is literally just a sum of all the hexadecimal character values in the file that gets loaded on the ECU, and it’s represented by a single “number” that also gets written to a field in the ECU tables. In that sense, it is kind of like an ID number.
The ECU uses this number as an error check to validate that none of the bits in the software are corrupted, both when writing a new flash to the ECU and also for self tests (often when first turned on). If a software or table bit gets flipped, or something is written wrong or corrupted, the checksum will add up to a different number. If it doesn’t add up, the ECU won’t attempt to load tables or start the car, essentially throwing an error.
The Smog Check machines are not scanning the whole ECU. They are simply checking the checksum value because it works like an ID. But it’s a bit more elegant than that, because it can’t simply be easily falsified or overwritten to a different value in a tune, as the ECU uses it for error checks.
Gotta hand it to them, as it’s a fairly simple way of checking a simple value that reflects the entire ECU flash tune pretty reliably.
Now, if someone like Cobb wanted to, they could probably rewrite the underlying Accessport software (not just the tune tables) to either use the value differently or relocate the actual checksum to a different area in the software, but it doesn’t seem like they’d be inclined to by policy these days, and it could be argued that the Accessport would then constitute an emissions defeat device, and nobody wants that.
From what I can tell, while the CARB checksum policy was announced several years ago, enforcement didn’t actually start until this January 2024, so it will be in effect going forward.
What you can probably safely do with a hybrid (arguably more safe than the stock tune but maybe not foolproof), is load a CARB legal Cobb stage 1 or 2 tune, and switch the tune to “Cobb Eco” mode, which will prevent the car from attempting to request more power than simple wastegate pressure, which will largely prevent higher boost levels where the hybrid behaves differently enough to not be “close enough” to match the tune.
