Fault detection in lambda probe

A measuring system comprises a lambda probe (SH) which is free of potential, a ground line (ML_SH) and a signal line (SL_SH). An offset voltage source (UOS) is connected with the ground line in order to raise the potential thereof to a predetermined value relative to ground. The output signal is amplified at G. The output signal is composed with a threshold voltage which is less than the offset voltage minus the maximum possible amplified negative voltage of the probe, a voltage below the threshold indicating a sensor fault. A pull down resistor RPD may be connected to the signal line.