APPARATUS AND A METHOD FOR DIAGNOSING AN EXHAUST GAS PURIFICATION SYSTEM

A diagnosing apparatus for diagnosing first and second vacuum switches and a throttle switch of an exhaust gas purification apparatus. The diagnosing apparatus includes first and second vacuum switches, a throttle switch, an engine speed sensor, and a computer. The second vacuum switch outputs a signal when the value of vacuum is greater than 80 mm Hg vacuum. The second vacuum switch outputs a signal when the value of vacuum is less than 350 mm Hg vacuum. The throttle switch outputs a signal when the throttle valve is open more than 9 degrees. The engine speed sensor detects an engine revolution number. A predetermined relationship is stored in the computer of the diagnosing apparatus and the relationship includes at least three ranges or areas. The computer determines whether the detected engine revolution number is within a first range when the output of the second vacuum switch is contrary to an output forecasted or predicted in accordance with the outputs of the first vacuum switch and the throttle switch on the basis of the predetermined relationship, whereby the diagnosing apparatus does not diagnose any of switches as out of order when the engine revolution number is within the first or range.