Bosch MED17.5.5 ECU Guide for VW, Audi, SEAT and Skoda 1.4 TSI and 1.4 TFSI Engines
A Passat comes in with the engine ECU already on the bench. The Bosch label says 0261S05728, the VAG sticker says 03C906016P, and the owner admits the car was “remapped by a friend” before he bought it. It now hesitates under light load and logs a lean fault now and then. Before anybody opens a map editor, you want three things: a full backup of what is in the unit right now, the stock file for that exact software version, and a clear picture of what the previous tuner changed. That is a typical Monday with the MED17.5.5, the Bosch petrol ECU behind a big share of VAG’s 1.4 TSI and 1.4 TFSI engines from roughly the Golf VI and Passat B6 era onwards.
Everything below is based on what we actually hold for this family: 363 published MED17.5.5 files in our catalogue, the tools they were read with, and the label numbers that came with them. It is written for someone with a unit on the bench and a customer waiting, so it sticks to identification, coverage, file types, reading and writing, and the faults that keep coming back.
How to identify a MED17.5.5 on the car and on the bench
On the car, start with a diagnostic scan. VCDS or ODIS will show the engine control module with its VAG part number and a four-digit software version, for example 03C906016AJ with version 9458. That pair is what you match files against, so write it down before anything else. The Bosch hardware number and the Bosch software number (the ten-digit 1037 number) are usually not both visible over standard diagnostics, so you get them from the label or from the ID read in your flash tool.
Where the unit sits depends on the model. On most of the Golf and Scirocco based cars it is at the bulkhead end of the engine bay, and on the larger platforms you may find it closer to the battery box. Check before you start pulling trim, because the plug and the bracket differ between platforms even when the box inside is the same.
On the bench, the label gives you the rest. The fields you care about are:
- Bosch hardware number. Always 0261S followed by five digits on this family. In our catalogue they run from 0261S04348 up to 0261S08293. On the label it is often printed with spaces, so 0261S05728 may appear as 0 261 S05 728.
- VAG part number. Nearly all of ours start with 03C, followed by 906016, 906027 or 906026 and a letter suffix (03C906016AJ, 03C906027BA, 03C906026BP). A few Audi units carry 03C997016 numbers instead, such as 03C997016A or 03C997016C. The 997 group is how VAG usually numbers exchange units, so treat those as the same family with a different parts history.
- Software version. The four digits that follow the part number (5715, 9660, 8913 and so on). Two units with the same part number and a different version are not the same file.
- Bosch software number. A ten-digit 1037 number, for example 1037504491. Newer units can show the later Bosch format instead: one Sharan file in our catalogue carries 10SW003887 rather than a 1037 number.
If the label is scratched or half missing, the ID read from KESS V2, PCM Flash or Flex usually fills in the gaps. We have a separate walk-through on reading these stickers across brands in how to read ECU label numbers.
The hardware number does not pick the file for you
This is the mistake we see most. The Bosch hardware number tells you which board and processor you have, not which software is on it. Our own data shows it clearly. Hardware 0261S05728 (10 files in our catalogue) turns up with 03C906016P 5715 and 03C906016N 5713 on the Passat, and with 03C906016AJ 9458 and 03C906016AJ 9971 on the Golf. Hardware 0261S06351 (9 files) carries 03C906016DJ 1441, 03C906016BD 8166, 03C906016BF 8172 and 03C906016DG 4768, all on Passats. 0261S07731 appears in a Passat CC with engine CTHD and in a Tiguan and a Sharan with CTHA. So when you search, lead with part number plus software version, then use the hardware number and the 1037 number to confirm.
Other hardware numbers that come up often in our catalogue are 0261S05808 (8 files), 0261S05812 (7), 0261S04653 (6) and 0261S06083 (6). If yours is one of those, the odds of an exact software match are better, but still check the version.
Not every MED17.5.x is a MED17.5.5
Bosch used several MED17.5 variants across VAG petrol engines, and the names get mixed up on invoices and in forum posts. Don’t assume a file labelled MED17.5 or MED17.5.2 will load into a MED17.5.5 because the digits look close. The tool’s ID read is the reference. If it says MED17.5.5 and the part number starts with 03C, you are in the right place.
Vehicles and engine codes in our MED17.5.5 catalogue
Almost everything we hold for this family is VAG group 1.4 petrol. By brand, VW accounts for 311 files in our catalogue (the shop lists them under both VW and Volkswagen, merged here), Audi for 42, SEAT for 5 and Skoda for 3. Audi calls the engine a 1.4 TFSI, VW, SEAT and Skoda call it a 1.4 TSI, and the ECU does not care which badge is on the boot lid.
The table below breaks the files down by model. Counts are the number of published files in our catalogue tagged with that model today. They are not a list of every car that ever left the factory with this ECU. A further 60 files are not tagged to a single model, so they don’t appear in the rows. Engine codes are the ones we see on the example files in this guide.
| Model | Brand | Files in our catalogue | Engine codes seen on our files |
|---|---|---|---|
| Passat | VW | 85 | CAXA |
| Jetta | VW | 43 | CAXA, CFBA |
| A3 | Audi | 38 | CAXC |
| Tiguan | VW | 37 | CAVA, CTHA |
| Scirocco | VW | 35 | CAVD |
| Golf | VW | 33 | CAXA, CAVD |
| Polo | VW | 8 | CTHE, CAVE |
| Sharan | VW | 6 | CTHA |
| Touran | VW | 6 | CAVB |
| A1 | Audi | 3 | CAXA |
| Octavia | Skoda | 3 | CAXA |
| Ibiza | SEAT | 2 | CAVF |
| Leon | SEAT | 2 | CAXC |
| Passat CC | VW | 1 | CTHD |
The engine code tells you more than the model name here, because the same Tiguan or Passat body was built with several 1.4 outputs. These are the codes attached to MED17.5.5 files in our catalogue, with counts:
- CAXA: 93 files, by far the largest group. Passat, Golf, Jetta, Octavia and A1 files all carry it.
- CFBA: 42 files. In our samples these are mostly Jetta reads done with KESS V2.
- CAVD: 36 files. In our samples these are mainly Scirocco and Golf units.
- CAXC: 36 files, the Audi A3 and SEAT Leon side of the family.
- CTHD: 26 files. CAVA: 23. CAVC: 20. CAVE: 11. CTHA: 10.
- Smaller groups: CAVB and CAVG with 6 each, CAVF, CNWA and CTKA with 5 each, CTHE with 4, CTHC with 3 and CMSA with 2.
Several of the higher-output codes (CAVD and CTHD are the ones most workshops will know) belong to the twin-charged 1.4, with both a supercharger and a turbo. That matters later when we talk about tuning and faults, because boost control on those engines has more parts that can fail, and a file won’t cover for a slipping supercharger clutch or a sticking bypass flap.
Example MED17.5.5 files from our catalogue
Here is a cross-section of real listings, chosen to cover different models, engine codes, hardware numbers and the two file types we hold for this family. The first column is the listing title exactly as it appears in the shop. Where the hardware column says “not listed”, the product was catalogued from the part number and software number only, which is normal for OBD reads where the label was never seen.
A few things jump out once you look at the list side by side. The Scirocco and the Golf both carry 03C906027BA, but on versions 9660 and 8913, on different hardware. The Leon, Octavia and 0261S08101 Polo rows show two 1037 numbers in the title, which usually means the unit was updated at some point and both the original and the current software numbers were recorded. The Sharan row is the only one here with a 10SW number. And the Ibiza file sits under a URL that says “vw-all”, which is just how it was first catalogued; the listing itself is a SEAT Ibiza with engine code CAVF.
When you compare your own read against one of these, match the part number and the four-digit version first. If those agree and the 1037 number agrees, you have the right file. If only the hardware number agrees, you don’t.
File types for MED17.5.5 and when a workshop needs each
For this family our catalogue holds two published file types today: original files (361 listings) and chip tuning files (2 listings). That split says a lot about how MED17.5.5 work goes in practice. Most jobs are repairs and returns to stock, not fresh tunes.
Original files
An original file is a stock read of the unit, as it came from VAG for that part number and software version. You need one when:
- An OBD write stopped halfway and the car no longer starts. A matching original is the cleanest thing to write back.
- A car was tuned and the owner wants it back to stock for a sale, a warranty claim or a dealer visit where the software will be checked.
- You suspect a previous tune but have no backup. Comparing the current read against a clean original shows exactly which maps were touched.
- You want to build your own calibration and need a known-good base rather than somebody else’s modified file.
- A replacement ECU turned up with the wrong software for the car, and you need to put the correct version on it.
Keep in mind that an original file contains the calibration and program, not the immobiliser data of the car it came from. Writing it restores the software, it doesn’t marry a used ECU to a different key set.
Chip tuning files
The two tuning files we hold include a Stage 1 for a Passat CC with engine CTHD, on hardware 0261S07731 and software 03C906026BP 5708. A Stage 1 file assumes a healthy, standard engine. Before you write any tune on a 1.4 TSI, check the basics: plugs and coil packs, boost leaks in the intercooler pipework, the diverter valve, and on higher-mileage cars the timing chain, which was a known weak point on this engine generation. Direct injection engines also build up carbon on the inlet valves, which no calibration will fix. If the car has a mechanical fault, a new file just makes the fault show up sooner.
Immo off and virgin files
We don’t currently have ready-made immo off or virgin listings published for MED17.5.5, although both exist as categories for other families. On this ECU most workshops solve a swapped or used unit by cloning the immobiliser data from the old ECU, which keeps the car’s security intact. Immo off is only for vehicles the customer owns and can prove they own, and only where it is allowed. Our immo off explainer covers what the file actually changes and the cases where it is the wrong answer. If you still need one for a specific software number, ask through the request form at the end of this guide.
Decat and rear lambda requests
Because this is a petrol ECU, DPF and EGR work doesn’t apply. The petrol equivalent that customers ask about is switching off the rear oxygen sensor monitoring after the catalytic converter has been removed or replaced with a sports item, usually to clear a P0420 fault. Be straight with the customer: removing or disabling the catalytic converter is not legal for road use in the EU, the UK and many other countries, and the car will fail an emissions test. It is for motorsport or off-road vehicles only. A P0420 on a road car is normally a failing cat, an exhaust leak or a lazy sensor, and it should be fixed as such.
Reading and writing a MED17.5.5: OBD, bench and boot
The tools recorded against our MED17.5.5 files give a good idea of what workshops use on this ECU. KESS V2 appears on 156 files and PCM Flash on 129, which makes OBD the normal route. Flex follows with 39, New Trasdata with 12 and KTAG with 11. Another 2 files were read with a New Trasdata and KTAG combination, and there is one file each from two other programmers.
OBD
On most MED17.5.5 versions you can read and write through the diagnostic socket with KESS V2 or PCM Flash, without opening anything. It’s quick, and the tool handles checksums on write. The catch is that an OBD read may not give you everything. Depending on the tool and the protocol, you may get the calibration area and not a complete image of the processor, and you won’t get the immobiliser data. An OBD backup is enough to put the car back on its own software. It isn’t enough to rebuild the unit if the flash is damaged.
Bench and boot
For a full backup, a clone, or a unit that no longer talks over OBD, take it off the car. KTAG, Flex and New Trasdata will read MED17.5.5 on the bench, and when the processor won’t answer in normal mode, boot mode lets the tool talk to it directly. That means opening the case and making contact with the boot points on the board, so have the correct pinout for your hardware number before you start and don’t pry the lid at an angle that bends the board. A full bench or boot read gives you the processor flash and the data area that holds the immobiliser and adaptation values, which most tools save as a separate EEPROM file. Keep both.
There is more background on choosing between the three modes, with MED17 examples, in our guide to OBD, bench and boot mode on EDC17 and MED17.
Cautions that save units
- Use a proper stabilised power supply, on the car and on the bench. A battery charger is not a power supply, and voltage dips during write are the most common reason a MED17.5.5 ends up dead.
- On the car, switch off everything that draws current: lights, blower, heated screen. Don’t let anyone open a door halfway through.
- Always take a full read before writing anything, and store it with the label photo and the VIN. If you only have an OBD read, say so in your notes.
- Check that the checksum is corrected before a file goes back in. Most flash tools do this on write, but a file edited outside the tool and written in boot mode may need correction in your editing software first.
- Some later VAG MED17 software is protected against modified writes over OBD. If your tool reports a protection error on a MED17.5.5, stop, read the tool’s notes for that software number, and plan on bench or boot mode rather than forcing it.
- Don’t mix files between versions. A file for 03C906016AJ 9458 is not a file for 03C906016AJ 9971, even on the same hardware.
Bench notes and typical MED17.5.5 problems
Car won’t start after a failed OBD write
Leave the ignition in the same state, keep the power supply connected, and try the tool’s recovery function first. KESS V2 and PCM Flash can often resume or rewrite a MED17.5.5 over OBD if the boot area survived. If the ECU no longer answers at all, take it to the bench and write a matching original in boot mode, then restore the data area from your backup. Our step-by-step for this situation is in recovering a bricked ECU after a failed flash. The part people skip is checking why it failed. More often than not it was voltage.
Used or swapped ECU, immobiliser active
A MED17.5.5 from a breaker will start the engine for a second and then cut, or not crank at all, with the immobiliser warning lit. The unit is paired to the car it came from. The usual fix is to read the data area from the car’s original ECU and write it into the replacement, provided the replacement has the same hardware number and you then load the correct software for the car. If the original ECU is completely dead and you can’t read it, the pairing has to be done through dealer-level adaptation. Check which route fits before quoting the customer a price.
Limp mode or boost faults after a tune
If the car goes into reduced power after a Stage 1 file, look at the file first and the car second. Was it written for exactly this part number and version? Then log the car with VCDS: requested versus actual boost, fuel rail pressure, and misfire counters per cylinder. On the 1.4 TSI, boost faults after tuning usually trace to a split hose, a tired diverter valve or a wastegate that was already marginal on the stock file. On twin-charged engines check the supercharger side as well. If the logs look clean, go back to the original file and see whether the fault follows the software or stays with the car.
Lean codes and hesitation on a car with an unknown history
This is the Passat from the start of the guide. Read the unit, then compare the read against a clean original for the same 03C906016P version. If maps were changed, you’ll see it. If the read matches stock, the problem is mechanical or sensor-related: air leaks after the throttle, a MAP sensor, injectors or low fuel pressure. Writing a fresh stock file onto a car with a vacuum leak changes nothing.
Two 1037 numbers, or none at all
Some titles carry two Bosch software numbers, like the Octavia file with 1037503708 and 1037531056. That usually reflects an update history, and your tool may show either one depending on what it reads. Match on the part number and version first. If your tool shows a 10SW number instead of 1037, that is a newer Bosch numbering style, not a different ECU family.
Questions workshops ask about MED17.5.5
Is the MED17.5.5 the same ECU as on the 1.8 and 2.0 TFSI?
No. Those engines use other MED17 variants, and files are not interchangeable between them. In our catalogue, the MED17.5.5 is essentially a 1.4 TSI and TFSI unit with 03C part numbers. Go by the tool’s ID read, not by the engine size someone wrote on the job card.
Can I read a MED17.5.5 over OBD?
On most versions, yes. KESS V2 and PCM Flash are the most common tools on the files we hold. For a complete backup that includes the immobiliser data, or for cloning, read it on the bench or in boot mode instead.
Will a file with the same hardware number fit my car?
Not necessarily. Hardware 0261S05728 alone appears with at least four different part number and version combinations in our catalogue. Match the VAG part number and the four-digit software version, and use the hardware number as a cross-check.
What if I have the right part number but a different software version?
Treat it as a different file. Later versions often carry calibration changes from service campaigns, and writing an older version may bring back faults VAG had already fixed. If we don’t list your exact version, send us the details and we’ll look for it rather than guessing.
Can I move a MED17.5.5 from one car to another?
Yes, if you clone it properly. Read the data area from the original unit, write it into a replacement with the same hardware number, and load the software that belongs to the car. Swapping the box without transferring the immobiliser data leaves you with a car that cranks but won’t run.
Will a Stage 1 file fix a 1.4 TSI that feels down on power?
Rarely. A car that has lost power on stock software has a fault, usually a boost leak, ignition parts or a sensor. Find and fix that first. A tune on a sick engine hides nothing for long.
Is an Audi unit with a 03C997016 number different from a 03C906016 unit?
It is the same ECU family. The 997 group is the way VAG usually numbers exchange units, and in our catalogue those Audi units sit on the same Bosch hardware numbers as the 906 ones: 0261S04653 appears with both 03C997016A and 03C906016G. What decides the file is still the part number and version your tool reads from the unit, so match on that.
When your MED17.5.5 software isn’t listed
With 363 files we cover a lot of the common versions, but there are more software numbers out there than any one catalogue holds. If you can’t find yours, send the Bosch hardware number, the VAG part number with its four-digit version, the 1037 or 10SW software number, the vehicle and engine code, and the tool you are reading with. A photo of the label helps. If you already have a full read, attach it, because it lets us compare against files we have and tell you quickly what matches and what doesn’t. Use the file request form, and have a look through the MED17.5.5 family page first, since new listings are added there as they come in.
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