- 1). Look at the instrument panel to see if the "Check Engine" or "Service Engine Soon" light is displayed. When a mass air flow sensor is malfunctioning, the engine computer detects a problem in one of the emission control system components and activates this warning light.
- 2). Start the engine and check for smoke coming out of the tailpipe. A bad MAF sensor affects the emission control system and will cause your car to run rich and expel smoke through the tailpipe. However, a defective MAF sensor also can cause the engine to run lean. You can determine if this is the problem by checking the tachometer to see if the rpm level is fluctuating when the engine is idling. Open the hood and check the spark plugs. If it is running lean, the spark plugs will be coated with white residue. On the other hand, if it is running rich, the spark plugs will be coated with black or gray residue.
- 3). Determine if the engine is running roughly, which can be a symptom of a defective mass air flow sensor. For instance, a bad MAF sensor will cause the engine to stumble and then recover when you start your car. It can also cause the engine to stall completely, requiring you to restart it. Other signs of a bad MAF sensor include delayed acceleration and and the engine idling roughly while sitting at a stop light.
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