Apparatus for diagnosing a spark ignition engine

The diagnosis circuit is for a motor vehicle engine ignition. Each cylinder has a spark plug connected via a potential cut off diode (20) connected with the secondary winding (14) of the ignition coil (12). The voltage (UZ) applied to the plug is compared to a preset value (US) by a comparator (30). The switching signal (UK) is evaluated as a signal fed to the fuel injector adjacent to the plug. The spark position is compared to a preset crankshaft rotational position.