A method is provided for diagnosing a reference channel of a broadband lambda probe that is used to determine an oxygen concentration in an exhaust gas, at least one sensor element having a pump cell and a Nernst cell being used; in one measurement mode, a regulated pumping current flowing through the pump cell to determine the oxygen concentration in the exhaust gas, and thus an exchange of oxygen ions between a measuring cell and the exhaust gas being achieved, and a lambda value in the measuring cell being regulated to a value of 1; the lambda value in the measuring cell being monitored through the Nernst cell, and the value of the pumping current required for that purpose being dependent on the oxygen concentration and thus on the lambda value of the exhaust gas to be determined.