The toxicity standards of modern cars are constantly tightening. Such standards can be reached only when using special systems and assemblies. The existing diagnostic methods are too time-consuming and do not exclude possible multiple errors. The article proposes a method for diagnosing the ICE (internal combustion engine) exhaust system of cars by the run-down time in test modes. An active study of possible diagnostic parameters allowed us to identify a sensitive diagnostic parameter -the run-down time. As the resistance of the catalytic converter increases, the run-down time decreases significantly. The experimental test showed the following: 1) when the run-down starts from 5,000 rpm, the run-down time in case of reaching the limiting resistance of the catalyst is halved -from 4.8 to 2.4 s; 2) in case of a limiting change in the resistance of the catalytic converter from 34 to 10 mm in the exhaust gases, the O2 content increases from 2.53 to 4.92 %, the CO2 content decreases from 13.17 to 12.57 %, and there is a sharp decrease in CH from 100 to 8 ppm. The run-down parameters control range is from 15 to 0.1 s. The production test of the new method for diagnosing the ICE exhaust system showed its significant performance.