Failure phenomenon: a 2007 year Audi Q7SUV Q7SUV, carrying VR6 3.6LFSI engine. The owner describes the engine fault warning light on the instrument, and the engine accelerates powerless. Fault diagnosis: the connection fault diagnosis instrument VAS5052 detects the engine system, and the engine control unit J623 has the fault code of "Holzer sensor G40 unreliable signal". After checking the line of the Holzer sensor, the fault diagnosis instrument was used to clear the fault code. The test vehicle found that the acceleration performance of the vehicle decreased obviously, and the engine fault light was lighted again after a distance of driving. Using fault diagnosis to detect and detect the fault code of "Holzer sensor G40 unreliable signal" again. There are only two possible reasons for the analysis and storage of Holzer sensor fault code: one is the Holzer sensor, the engine control unit or the related line fault; two is the engine valve timing error or the Holzer sensor target wheel installation position is incorrect. The VR6 engine is equipped with 2 parts of the same Holzer camshaft position sensor G40 and G163, previously checked the line of the Holzer sensor G40, adjusting the position sensor of the camshaft, clearing the fault code test, accelerating the failure of the failure still exist, G40 can not rely on the fault code will appear. The intake manifold, high pressure fuel guide and valve chamber cover are removed to check the normal timing of gas distribution, but it is found that the camshaft chain tension is insufficient and the activity is large. It was found that the fixed bolt of the sprocket used to connect the crankshaft chain and the camshaft chain was broken and the sprocket and chain was loosened and the gas phase was over. Because the design of the engine is compact, the rear timing cover is closer to the sprocket and has a groove stuck to the sprocket, so the sprocket does not fall off. The user has been driving for some time before coming to the factory, and the inside of the timing cover has been worn out by a dent, so the engine has no obvious abnormal noise. Troubleshooting: change timing chain, sprocket, sprocket fastening bolt and rear timing cover, install trial run to determine troubleshooting.