PURPOSE: To detect the fault of an exhaust gas oxygen sensor, distinguish a property downward deviation fault from the other signal fault of the sensor and apply a front-end operational amplifier.

CONSTITUTION: In the method and device of detecting the fault of an exhaust gas oxygen(EGO) sensor, the first signal is issued, which is provided with the first voltage if an EGO sensor output signal approaches to non-allowable voltage and with the second voltage if the EGO sensor output signal gas non-allowable voltage. The voltage of the first signal is measured at the first clock and, if the measured voltage has the first voltage, the first flag is set. The first signal is measured at the following clock, inspection that the first signal has the second voltage and the first flag is set is made and, if the detection result is affirmative, the second flag is set to indicate the fault of the EGO sensor.