A BMW 525i sedan, driving weak and exhaust pipe a lot of white smoke, dashboard "CHECKENGINE" fault indicator light. Read the error code 1215, meaning the air flow meter or its line bad. After testing, the air flow meter impedance does not meet the requirements. Replace the air flow meter after the test, the situation slightly improved, but the fault remains the same.
     Check again and found that the 4th cylinder spark plug is covered with gasoline, indicating that the cylinder does not work. Infer the fuel injector has a problem, replace the nozzle after the test, no fault code appears, but the condition remains the same. With LED lights detection and found that the engine computer's fourth cylinder fuel injection signal output line no signal (good line), concluded that the computer is faulty. Replace the engine computer, troubleshooting.