How to check you have the right original before writing it

An original file only helps if it is the right one. Writing software of a different version, a partial file into a full-memory write, or a file with wrong checksums can leave a vehicle that does not start and an ECU that has to be recovered. Checking takes a few minutes and needs nothing more than the numbers, your own read and a program that can compare two files.

This checklist is split into two parts: what to check before you buy, and what to check before you write.

Before you buy: match the numbers

Save your own read first

Whatever you plan to do, read the ECU and keep that file safe before anything else. It is your way back, and it is the reference you will compare the original with. If your tool offers several read modes, keep every file it produces.

Get the numbers from the ECU, not only from the label

Note the HW number, the SW number and, if available, the calibration ID — ideally from diagnostic identification or from your read. The label shows what was fitted at the factory; after a dealer update the software inside can be different. Where to find each number is explained in how to find the HW and SW number.

Compare with the file page

• SW number / calibration — must match. This is the deciding check.
• HW number — should match. Several files sharing your HW number is normal; the SW number picks one.
• ECU type — must be the same type (for example EDC17C46, not EDC17C64).
• Vehicle and engine — should be plausible. The same software is sometimes used in several models, so a different model name is not by itself a problem; a different engine family is.

All sales are final.Files are digital and delivered by e-mail, so they cannot be returned.

After download: compare the original with your read

Open your own read and the original side by side. WinOLS can load both into one project and list the differences; most hex editors also have a file-compare function. Look at four things.

1. Identification strings

Search both files for the SW number (in the text/ASCII view of a hex editor, or in WinOLS's identification). The same number should appear in both, at the same place.

2. Size and layout

Compare the file sizes and the overall picture: where the filled areas are, where the empty (FF or 00) areas are. A different size does not automatically mean a wrong file — one may be a full read and the other only the calibration area, or one tool may pad unused memory and another not. Find out which before you continue; why ECU file sizes differ lists the usual reasons.

3. The amount of difference

• Same version, both stock: no differences, or only a few small areas (counters, unit-specific data, areas a particular read method does not capture).
• Same version, your read modified: differences concentrated in the calibration area — map regions — while code and identification match. This is the typical case where an original is used to go back to stock.
• Different versions: differences spread across the whole file, identification strings differ. Stop here — this is not the file for your ECU.

4. Byte order

If almost every byte differs but the files look alike when you swap each pair of bytes, one of them was saved in a different byte order (this happens with some 16-bit memory chips and read modes). Convert one of them before comparing again, and make sure the file you write is in the order your tool expects.

Before you write

Does your tool accept this file?

Writing tools differ in what they accept. Some write only the calibration area over OBD, some write the full memory on the bench or in boot mode, and some only write files that come from their own read session or their own file format. Check in your tool's documentation how an external original is loaded for writing — for example, whether it has to be imported into the session of your own read.

Checksums

An unmodified original carries the checksums that belong to its own content. If you change even one byte, or combine parts of two files, the checksums must be corrected before writing. That is done by your writing tool or by the checksum modules of your editing software — never by hand-editing values and hoping. Check how your tool handles checksums even when you write a file unchanged.

Power and patience

Use a stable power supply (a battery support unit, not a weak battery or a charger with ripple), keep the ignition and the connection undisturbed, and do not interrupt a write. Most failed writes have trivial causes.

Keep the evidence

Keep your original read, the file you wrote and the tool's log until the vehicle has been driven and checked. If something goes wrong, those three files are what anyone helping you will ask for first.

The short version

1. Read the ECU and save the read before anything else.
2. Take HW, SW and calibration numbers from diagnostics or the read, not only the label.
3. Buy only when the SW number (or calibration ID) and the ECU type match the file page.
4. Compare the original with your read: identification, size and layout, amount of difference, byte order.
5. Make sure your tool accepts the file and handles the checksums, then write with stable power.

Search by number with the search box at the top of the page, or browse by make and ECU type. Every original file costs €18.