A method/control unit for detecting a voltage offset in a range of a voltage-lambda characteristic curve of a two-point lambda sensor in an engine exhaust gas duct as to a reference voltage-lambda characteristic curve of the two-point lambda sensor, which is part of a controlled system for adjusting an air-fuel mixture supplied to the engine, a characteristic curve deviation of voltage-lambda characteristic curve to the reference characteristic curve being corrected at λ=1, so that a change in the air-fuel mixture composition toward λ=1 occurs, starting from a value pair to be checked on the reference characteristic curve, with a lambda and a voltage to be checked, the actual value of lambda being inferred from the change in the mixture composition until reaching λ=1. The method/control unit permit determining and compensating a voltage offset of a two-point lambda sensor conditional upon aging or manufacturing tolerances.