A FAW Volkswagen Bora 1.6 L manual car with a mileage of about 120 thousand km. The engine of the car has been overhauled, it has difficulty in starting up, and it is normal at idle speed, but it is unable to accelerate its energy consumption and fuel consumption. After carrying on the car, the basic inspection is carried out first. The engine is working normally at idle speed and runs smoothly. Next, the fault code is retrieved by VAS5052 and the fault code stored in the camshaft position sensor (G40) is found. After checking the line without exception, the G40 was replaced, but the test found that the above fault still exists. Again, the fault code is still G40 failure. It is doubted that the timing belt is incorrect, but there is no abnormality in the inspection of the timing belt. The fault is a little confused, because the ignition and fuel injection of the car have been checked, all of which are normal. Because of the frequent display of the G40 fault, it was found that the timing mark was correct only when the valve chamber cover was removed again to check the chain timing mark behind the camshaft, but it was found that when the gear was rotated, the clearance between the chain tensioner up and down was too large and the fault was determined by the damage of the chain tensioner. Because of the damage of the chain tensioner, the G40 sometimes gives the engine ECU a wrong signal, which causes the ignition time of the engine to be delayed and the engine accelerates. Elimination method: replace chain tensioner.