The 91 E34 uses OBC 4. To deactivate this in the event the code is forgotten do the following;
1. Disconnect and then reconnect the battery
2. Turn ignition to position 1 only. (the horn may/will sound)
3. The OBC will now display a time of 15:00 minutes, this will now count down.
4. After the 15:00 minutes has elapsed the car may be started.