Method for operating a glow plug

The method involves generating an effective voltage from a vehicle electrical system voltage, where the effective voltage is applied to the glow plug. A set of values of combustion chamber pressure (1) in a cylinder is measured during each working cycle of the cylinder in relation to an angular position of a crankshaft. A characteristic value of a combustion process is determined from an evolution of the combustion chamber pressure measured. The characteristic value is compared with a setpoint value. The effective voltage is set to a minimum value required for reaching the setpoint value.