Checking whether an ECU file is stock or modified

Check Stock or Not tells you whether an ECU file you have read still holds the factory software or has already been modified. It is worth doing before a tune, when buying a used car and whenever a customer dispute comes up. For tuners: send us the file and get the answer, on average within 30–60 minutes.

When a stock check makes sense

A car arriving with someone else's tune is no longer unusual. HP Academy estimates that in some regions up to 40% of cars presented for tuning already carry a modified calibration, compared with roughly 10% about ten years ago. Typical situations where the check pays off:

• Before tuning. A tune is built on the original file. If the ECU already runs another tuner's Stage 1 or a DPF delete, your changes end up stacked on top of theirs: limits raised twice, some faults already switched off, and no clean baseline to work from.
• Buying a used car. The buyer wants to know about earlier tuning and disabled systems. The seller may not mention either.
• Customer complaints. A car comes back with a problem blamed on "the remap". The first step is finding out what is actually in the ECU now.
• Dealers and software checks. Dealers compare the software version and CVN, and since 19 July 2021 California's Smog Check does the same: a car with modified software fails with the result "Modified Software" and has to go back to factory or CARB-approved software.
• Before a return to stock. If the customer kept an "original", make sure it really is the factory file and not somebody's tune saved under the name ori.

What an ECU file contains and where edits leave traces

The file a tool reads is the content of the ECU's flash memory: bootloader, program code and calibration. The calibration holds the fuel, boost and torque maps, the limiters and the fault table. The same memory carries the identifiers (hardware number, software number, CAL ID) and the checksums the ECU uses to verify its own data.

A tune nearly always changes the calibration and sometimes the code as well, with patches or disabled checks. The identification strings are normally left alone, so the software number of a tuned file looks exactly like stock. Vehicle-specific data such as the VIN, immobiliser data and adaptations usually sit separately in EEPROM or DFLASH. They differ from car to car, which is why they are never compared with a reference file.

How to tell whether a file is stock

Signs on the car itself only give you a reason to look closer. An aftermarket intake or exhaust does not mean the software was changed, and power above the factory figure on a dyno is a strong but indirect hint. The file gives the real answer, and even there no single sign is enough: each check looks at its own part of the file, and each has a blind spot.

What a scan tool shows without reading the ECU

Over OBD (mode 09) the ECU reports its CAL ID, which names the software version, and its CVN, a check value the ECU calculates itself from its memory content. Dealers compare both with the factory data for that VIN. A GM dealer bulletin spelled out the logic back in 2008: if the calibration numbers match but a CVN does not, the calibration in the ECU is most likely not GM's. California's inspection works the same way: any change to the program produces a new CVN that no longer matches the value tied to that CAL ID.

Why a matching CVN does not prove the file is stock

A CVN can be corrected. A compensation value is added to the file and the ECU reports the factory number again; tuners mention it openly in their write-ups ("the CVN stayed the same"). So a matching CVN means one of two things: the file is stock, or it was edited carefully. Only a comparison with the original tells them apart. You can check the CVN of your own file in the CVN correction section.

How the check works

The file is identified by its hardware and software numbers first, then compared with the original of the same software version. No differences means the file is stock. If there are differences, you can see exactly where: in the maps, in the fault table or in the code. When no reference exists for that version, the verdict is based on checksums, CVN and typical traces of tuning. That is still useful, but less certain than a byte-by-byte comparison.

What a modified file usually reveals

• Edited maps: torque limiters, boost, fuel quantity, ignition timing on petrol engines, the smoke limiter on diesels. That is what a Stage 1 or any "chip" looks like.
• Disabled faults and functions: traces of a DPF, EGR, AdBlue or lambda delete, usually in the fault table and in the calibration of those systems.
• Code patches: removed write protection (unlock), disabled checks.
• CVN compensation: a few bytes that bring the CVN back to the factory value although the content has changed.

When the changes are limited to one system, they tell you what was done to the car. Edits only in the EGR area with factory power maps, for example, point to an EGR delete without a tune.

Which file to send

• A real read, ideally a full one. Over OBD, some tools only read the areas needed for tuning. Autotuner, for example, saves a backup of all memory areas only on a bench or boot read.
• Not a virtual read. Some ECUs cannot be read over OBD, so the tool downloads the original matching the software number from its server instead. Such a file is always stock, even on a tuned car, and says nothing about what is actually in the ECU. If the ECU cannot be read over OBD at all, only a bench read gives you the real file, sometimes with the housing opened.
• ECU numbers. If the tool showed the hardware and software numbers, or you have a photo of the label, add them to the order. It makes the reference easier to find.

What to do with the result

If a file was edited without correcting the checksums, they will not match. Do not flash it like that: checksums are corrected before writing, which you can do in the checksum correction section.

How to order the check

1. Read the ECU with the tool you use, preferably a full bench read.
2. Upload the file to the portal and select CHECK STOCK OR NOT. Add the ECU numbers to the order if you have them.
3. Get the answer on average within 30–60 minutes: stock or modified.
4. Decide on the next step: tune from the stock file, or return to stock first.