The mileage is about 120 thousand km, the 2010 Audi Q5 2.0TFSI sports utility vehicle equipped with CADA engine and 0B5 automatic transmission. User response: the engine jitter is serious and the vehicle is powerless. Inspection and analysis: the maintenance personnel found that the car was shaking and idling unsteadily. The engine control unit is detected by VAS5052A, and the fault code 00772-4 cylinder is found to be on fire. Check the 4 cylinder ignition coil and spark plug. No abnormalities were found. After interchanging with the 1 cylinder ignition coil and spark plug, the fault still exists. When the idle speed is raised, the phenomenon of fire loss is relieved. Check engine control unit to ignition coil and fuel injector connection. Measuring the cylinder pressure of the 4 cylinder was found to be only 0.4 MPa, far below the normal value. Removing the cylinder head found that there was a gap in the 4 cylinder exhaust valve, which caused the cylinder seal to be lax, resulting in a 4 cylinder fire. Further examination revealed that the 4 cylinder exhaust valve spring was fractured. Troubleshooting: replace valve and reset spring, repeat cylinder pressure, change to 1.2 MPa, normal. Troubleshooting the test car.