A Mercedes Benz E200 car with a mileage of about 80 thousand km and a 642 diesel engine. It runs on 2010. User reflection: the car engine failed to light. Inspection and analysis: maintenance personnel detect engine control unit and find fault code P2355---- exhaust gas recirculation system (EGR) failure. For diesel engines, the exhaust gas introduced by the EGR system enters the engine under the same air pressure as the air intake, which is completely different from that of the gasoline engine. Therefore, at the same engine speed, exhaust gas and air always enter the cylinder at a certain proportion. In this way, air flow can be used to understand the working status of the EGR system. When the engine is idle, the air flow rate is observed to be obviously higher than the normal value. This shows that the ratio of air to exhaust gas is abnormal, that is to say, only a small amount of exhaust gas enters the engine. This is the reason why the engine lights up. The pipeline inspection of the EGR system was removed and found to be blocked. Careful observation shows that the inner wall of the pipeline has collapsed, resulting in poor gas flow. The cause of the trouble is analyzed, and the maintenance staff think it is caused by the failure of the exhaust gas cooling system. Troubleshooting: after replacing the exhaust gas cooler and exhaust gas recirculation line, the air flow at idle speed is observed again, and the data is normal. Troubleshooting the test car.