Control circuit for operating a diesel engine

The method involves detecting the energization of a diesel injector (5) with control current by a gas fuel control device (25). The supply of the gas fuel (31) with a supplying device is controlled by the gas fuel control device in response to the energization of the diesel injector. The energization of the diesel injector with control current is interrupted by the gas fuel control device so that the actual quantity of the diesel fuel (30) supplied to the combustion chamber (4) is reduced due to the interruption of the control current. Independent claims are included for the following: (1) a method for retrofitting a diesel engine; and (2) a retrofit system for a diesel engine.