The invention relates to a method (100) for operating a solenoid valve (1) for metering a fuel (2) in a fuel injection system (3). The solenoid valve can be actuated against a restoring force (12) by an electromagnet (11), wherein •the time curve l(t) of the current I flowing through the electromagnet (11) and/or the time curve U(t) of the voltage U applied to the electromagnet (11) are detected during at least one opening process of the solenoid valve (1). The opening time tON and the closing time tOFF of the solenoid valve (1) are evaluated (110) from the time curve I(t) and/or U(t), and the actual opening duration TT=tOFF−tON of the solenoid valve (1) is compared (140) with a reference value TR and/or •the mass flow dm/dt flowing through the solenoid valve (1) is detected (120) and compared (142) with a reference value MR during at least one opening process of the solenoid valve (1); and/or a leakage dm′/dt of fuel (2) through the solenoid valve (1) is detected (130) in the closed state of the solenoid valve (1). The invention also relates to a corresponding controller (5), a fuel injection system (3), and a computer program product.