HW and SW, put simply
When you read an ECU with your flashing tool — a Kess V3, an Autotuner or a KTAG — two numbers always come up: the HW (Hardware Number) and the SW (Software Number). Both are fundamental before any remap.
HW — the Hardware Number
The Hardware Number identifies the physical revision of the control unit. It is the number of the board itself: the components, the printed circuit, the memory chips. That number never changes — no matter how many times you remap the ECU, the HW stays exactly the same.
SW — the Software Number
The Software Number identifies the version of the program loaded into the ECU. It is the number of the factory calibration — the code that runs injection, ignition and boost. This is the number that changes when you remap an ECU.
When a manufacturer releases a software update — to fix a fault, improve economy or meet a new standard — it is the SW that changes. The HW stays put.
Why it matters so much when remapping
The HW/SW pair is what identifies an ECU exactly. Finding the right Damos file, the right map, or simply avoiding a bricked unit all depend on knowing both numbers.
- The same ECU can carry several different SWs depending on country, market or model version. A file written for SW 8623A02 will not work properly on SW 8623B01.
- The HW determines physical compatibility with flashing tools. Some protocols only work on certain hardware revisions.
- Damos files are indexed by HW/SW — that is how you find the right one in a database holding tens of thousands of them.
How to read the HW and SW of your ECU
There are two ways:
With your flashing tool
The most reliable route. Plug the tool in over OBD (Kess V3, Autotuner, KTAG…), pick the vehicle and start a read. The software shows the detected HW and SW before it even pulls the full file.
With an OBD diagnostic tool
A plain diagnostic tool (VCDS for Volkswagen/Audi/Seat/Skoda, ISTA for BMW, Techstream for Toyota…) shows the ECU part numbers under "control unit information". The format varies by brand, but the HW and SW are always there.
Number formats, brand by brand
| Brand / ECU | HW format | SW format | Example |
|---|---|---|---|
| Bosch (generic) | X XXX XXX XXX | X XXX XXX XXX | HW: 0 261 S12 345 |
| Volkswagen / Audi / Seat / Skoda | XXX XXX XXX X | XXX XXX XXX X | HW: 03C 906 026 BM |
| BMW (Bosch DME) | XX XXXX XXXX | XX XXXX XXXX | SW: 12 14 7 638 123 |
| Ford (Delphi/Bosch) | XXXXXXXXXXX | XXXXXXXXXXX | SW: 3M5112A650PB |
| Renault / Peugeot / Citroën | Varies | Varies | Depends on the ECU |
The checksum — the third number
Alongside the HW and the SW you will often see a third number: the checksum. It is a value calculated from the contents of the ECU file — a kind of digital fingerprint used to confirm the file is not corrupted.
As soon as you modify a map, the checksum changes. Save the file with the wrong checksum and the ECU will refuse to start — one of the most common causes of a bricked unit. WinOLS and most remapping software correct the checksum automatically, but it is a critical step and never one to skip.
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In short
HW is the number of the physical hardware, and it never moves. SW is the number of the software, and remapping changes it. Both are essential to identify an ECU precisely, find the right Damos file and flash safely. Before any remap, the first step is always to write them down.