Bosch M1.7.2 Immo Off for Opel Corsa B, Astra F and Vectra A: Chip-Level Guide

The Bosch Motronic M1.7.2 (also labelled M1.7 and M1.7.1) controls the 1.6 8v/16v petrol engines in the Opel Corsa B, Astra F, Vectra A and Calibra built from 1993 to about 2000. These ECUs pre-date OBD flashing: the software lives on a socketed or soldered flash/EPROM chip, and immobilizer problems — dead Immobilizer II unit, lost transponder keys, used ECU — are solved with a modified chip image. Here is how it works.

Which ECU do you have

The label carries the Bosch number 0 261 203 xxx or 0 261 204 xxx and the GM number (90 xxx xxx). The important detail is the chip: early units use a 27C512 EPROM, later ones a 29F010 or 28F010 flash. The file we deliver is the full chip image for that memory type and for your GM part number.

When immo off is needed

• The Immobilizer II control unit (behind the dashboard) has failed and a used one cannot be paired without the original key card.
• All transponder keys are lost.
• A used ECU from another car refuses to start the engine.
• Engine swaps into cars that never had the immobilizer.

Reading and writing the chip

There is no OBD path on the M1.7.2. Open the ECU (four screws, then the side clips), locate the memory chip next to the processor and read it with a chip programmer: XPROG, UPA-S, TL866 or any device that supports 27C512 / 29F010. If the chip is soldered, either desolder it or use a clip adapter. Save the original image first.

1. Read and verify the chip; note the exact device type.
2. Write the immo off image to the same chip or to a new blank chip (29F010 for flash units, 27C512 EPROM for the old ones).
3. Verify after writing, refit the chip with the notch in the right direction and close the ECU.
4. Start the engine; the immobilizer LED may keep blinking but the engine runs.

What changes in the file

Only the immobilizer routine is disabled. Fuel and ignition tables are untouched, so the car drives exactly as before. Because the M1.7.2 has no checksum protection in the immobilizer area, the image does not need a checksum correction — but it must match the correct GM part number, otherwise the ECU may run a calibration made for a different engine variant.

Tips from the workshop

• Take a photo of the chip orientation before removing it.
• EPROM chips (27C512) can be erased only with UV light; use a new chip if you have no eraser.
• If the car still does not start after the immo off, check the crank sensor and the fuel pump relay — both are common failures on cars of this age.

Getting the right file

Our immo off catalogue for Opel lists M1.7.2 images by GM and Bosch number. If yours is not there, read the chip, upload the image on the file request page and we return the modified version.