Siemens Simos Immo Off: VW, Seat and Skoda 1.4 / 1.6 Petrol Simos 3.3A, 7.1 and 9.1 ECU Guide

While Bosch ME7 units get most of the attention, the small VAG petrol engines run on Siemens Simos ECUs: Simos 2.1 (1.4/1.6 8v Golf IV, Polo 6N2), Simos 3.3A / 3.4 (1.4 16v AXP, BCA), Simos 7.1 / 7.1A (1.6 FSI and 1.4 16v Polo 9N, Golf V, Fabia, Ibiza 6L) and Simos 9.1 (1.6 BSE/BSF Golf V, Octavia II, Leon 1P). A Simos immo off file is the usual rescue when one of these cheap, plentiful cars receives a used ECU or loses its cluster.

Immobilizer behaviour

Immo 2 (until ~2001) or Immo 3 (from ~2001): the cluster holds the keys and the ECU stores the pairing. A used Simos starts the engine for a second and stalls, immo lamp flashing, fault 17978 / P1570. On Immo 3 cars the 7-digit SKC of the cluster is needed to pair a used ECU; without it, a virgin or immo off file is the way.

Immo off or virgin?

• Virgin — the ECU is reset to unpaired; VCDS adaptation with the cluster SKC then pairs it normally. Keeps the immobilizer for a road car.
• Immo off — the ECU no longer checks the cluster. For engine swaps, dead clusters and cars converted to aftermarket dashboards.

Reading the Simos

1. OBD: Galletto 1260, MPPS and KESS V2 read Simos 2.1, 3.3A and 7.1 over K-line; Simos 9.1 over K-line/CAN depending on year. Battery on a charger — a dropped write leaves the ECU in bootloader.
2. Bench: KTAG and Flex read all Simos generations through the connector, including the EEPROM where the immo data sits on later versions.
3. Note the VAG number (036906034xx, 03C906024xx, 06A906033xx) and the Siemens number 5WP4xxxx; the software version must match the file.

After the write

Immo off: the engine starts and stays running immediately. Virgin: run the adaptation in VCDS (Engine → Login with SKC → Adaptation channel 50) before expecting a start. If the engine starts and stalls even after a correct file, check that the cluster was not swapped as well — two unpaired components at once confuse many diagnoses.

Common pitfalls

• Simos 3.3A and 3.4 files are not interchangeable despite identical connectors.
• Simos 7.1 (1.4 16v BBY/BKY) and Simos 7.1A (1.6 FSI BAG/BLF) are different units with different files.
• A Simos 9.1 with EPC lamp and no throttle response after ECU replacement needs a throttle body adaptation (basic setting), not another file.

Files

Browse Simos 3.3A, Simos 7.1 and Simos 7.1A files, or start from VW, Seat and Skoda. The ME7.5 of the 1.8T and 2.0 is covered in our ME7.5 immo off guide. Missing number? Use the file request form.