In a method for diagnosing an injection valve (5), in an overrun fuel cut-off phase the fuel supply to the fuel rail (4) is closed, and after the fuel supply has been closed off a first fuel pressure in the fuel rail (4) is measured, and after the first measurement of the fuel pressure an injection valve (5) is actuated for a test injection. After the test injection a second fuel pressure in the fuel rail (4) is measured, a differential pressure value (ΔP) is formed from the first and second measured fuel pressures and a difference of an operating parameter from a reference parameter is determined from the differential pressure value (ΔP), and when a previously defined maximum difference is exceeded the injection valve (5) is detected as being defective.