PURPOSE: To prevent the abnormal operation of an engine by a method wherein a power supply means is provided for supplying power to an electromagnetic valve, which controls the air flow rate, when any trouble occurs to interrupt the power supply to the electromagnetic valve.

CONSTITUTION: In addition to a main drive circuit 36, an auxiliary drive circuit 48 is provided in a circuit controlling the power supply to the winding of an electromagnetic valve means 10 which comprises a body 12, valve member 14, electromagnetic winding 16 and a spring 18. The circuit 48 is connected to one end of the winding 16 through a line 50, the other end of the winding 16 connected to a power source B through contact rl2 of a relay RL. With this construction, when any trouble is detected in the wiring of the circuit 36, such as disconnection as indicated by references p and p' in the circuit diagram, a central processor 32 in the circuit 30 selects the circuit 48 to ensure the operation of the valve 10.