Possible Causes

Cylinder reference sensor or supply wires defective. See Figure .
Cylinders No. 6 or 12 receives no ignition voltage signal.
No spark-over at the spark plug of cylinders No. 6 or 12.
Ignition coil defective.
Ignition output stage of DME control unit defective. See PULSE GENERATOR - CYLINDER REFERENCE (IGNITION) .