A fault detection method with a precondition that an intake temperature on an entry side of an intercooler, an ambient temperature, an EGR gas temperature on an entry side of an EGR cooler and a cooling water temperature into the EGR cooler have been determined to be normally detected, which comprises calculating intercooler efficiency on the basis of the intake temperature on the entry side of the intercooler, an intake temperature on an exit side of the intercooler and the ambient temperature, calculating EGR cooler efficiency on the basis of the EGR gas temperature on the entry side of the EGR cooler, an EGR gas temperature on an exit side of the EGR cooler and the cooling water temperature, and determining whether a calculated value of an intake temperature at an intake manifold on the basis of a current EGR ratio, the EGR gas temperature on the exit side of the EGR cooler and the intake temperature on the exit side of the intercooler deviates from an actually measured value thereof or not, and identifying a fault place in an intake system by comparing three elements mentioned in the above, i.e., the intercooler efficiency, the EGR cooler efficiency and any deviation of the calculated value of the intake temperature at the intake manifold from the actually measured value, wherein, on condition that a calculated value of the intercooler efficiency is outside normal limits and a calculated value of the EGR cooler efficiency is within normal limits, if the calculated value of the intake temperature at the intake manifold deviates from the actually measured value thereof, an intercooler exit temperature sensor for detection of the intake temperature on the exit side of the intercooler is determined to have fault whereas if the calculated value of the intake temperature at the intake manifold does not deviate from the actually measured value thereof, the intercooler itself is determined to have fault; on condition that the calculated value of the intercooler efficiency is within the normal limits and the calculated value of the intake temperature at the intake manifold deviates from the actually measured value thereof, if the calculated value of the EGR cooler efficiency is within the normal limits, an intake manifold temperature sensor for detection of the intake temperature at the intake manifold is determined to have fault whereas if the calculated value of the EGR cooler efficiency is outside the normal limits, an EGR exit temperature sensor for detection of the EGR gas temperature on the exit side of the EGR cooler is determined to have fault; and on condition that the calculated value of the intercooler efficiency is within the normal limits and the calculated value of the intake temperature at the intake manifold does not deviate from the actually measured value thereof, if the calculated value of the EGR cooler efficiency is outside the normal limits, the EGR cooler itself is determined to have fault whereas if the calculated value of the EGR cooler efficiency is within the normal limits, all of the intercooler exit temperature sensor, the intercooler, the intake manifold temperature sensor, the EGR exit temperature sensor and the EGR cooler are determined to be normal.