The probe is arranged downstream of a catalytic converter, and is sensitive to one or more exhaust constituents. The rate at which the signal from the probe reacts when the concentration of the constituent is changed, is used as a judgement criterion for diagnosing the probe performance. The probe may be an oxygen-sensitive probe with Nernst characteristics. The reaction of the probe to changes in the oxygen content of the exhaust gas is evaluated after the start of shifting phase with fuel switch-off. Faults in the probe are displayed or stored.
