1991 BMW 735i Mass Airflow Sensor Replacement Cost

The engine's computer has to receive input from many sensors, and ensure they are in agreement. An extremely important sensor that is used in reference to nearly all engine operations is the mass airflow (MAF) sensor. The MAF sensor records the volume of air that enters the engine's intake tube, and reports that information to the computer. This allows the computer to calculate, or confirm, the amount of air moving into the engine, amount of exhaust gas to expect, the amount of fuel to send into the engine, and many other important factors. Lastly, this sensor works very closely with the manifold absolute pressure (MAP) sensor to allow extremely accurate response to changing conditions.

If the MAF sensor sends an erroneous signal, a few things happen. The engine's computer will trigger the check engine light, and the on-board diagnostic (OBD) trouble codes will reflect the airflow discrepancy noted by the MAF sensor. Depending on the reported volume of air, the engine may try to compensate for these conditions, causing the engine to burn excessive amounts of fuel and emit black smoke from the exhaust. In this case, the OBD trouble codes would also reference faults recognized by the exhaust oxygen sensors. The engine may also have issues with idling roughly, failing to start, stalling, hesitation, power loss, misfires, and fuel consumption. »MORE: Signs of a failing mass airflow sensor

When the MAF sensor fails driving can be dangerous. As engine power will surge on and off, the vehicle may lurch forward. Overall engine performance may become unpredictable and difficult to control. It is preferred to tow the vehicle if any of the mentioned symptoms are present. If the vehicle operates normally, or near normally, driving the vehicle should not cause any issues.

MAF sensors do tend to fail sometime in most vehicles. Failure rates are highest between 100,000 and 125,000 miles. The sensor can also be damaged during air filter changes, and anytime the engine intake is serviced. Lastly, a collision may cause a physical or electrical fault in the MAF sensor.

When the check engine light is on, and OBD trouble codes refer to the MAF sensor, the servicing technician will begin by inspecting the vehicle for any indications of physical damage before moving on with testing. A computerized scanner will be connected to the vehicle, and current conditions will be compared to current readings from the various sensors in the vehicle. When these conditions are compared and analyzed, the technician relies on their classroom training to understand which component to test. Once one or more components are suspected of failure, the servicing technician will begin testing with the most likely component, until the issue has been verified through testing.

Replacing the MAF sensor is generally quick and easy. The sensor will typically be on or adjacent to the intake air tube, and the fastener may be a screw or clip. Once the sensor is replaced, the technician will review sensor data, and confirm the issue has been resolved.

We always recommend contacting the dealership parts department for revised part numbers. There may be a new part number for the mass airflow sensor, which can be a redesigned part that fixes known flaws. Aftermarket sensors may not have these benefits.

There is a common misconception that when an OBD trouble code is read, the component listed in the code should simply be replaced. This isn't always true, and can lead to expensive and unnecessary repairs. The codes actually refer to engine conditions being reported by sensors. Generic code readers are limited by their minimal programming, and typically are not advanced enough to view and test vehicle specific systems. A "MAF Sensor" title in a code does not necessarily mean the sensor has failed, but rather gives the technician a guide on where to begin their diagnosis.

Almost anyone can replace a MAF sensor. They are typically removed with a clamp, or a screw, and have one electrical connector that can be a little tricky on some models. Otherwise, protecting the sensor while working is all there is to it, and the some vehicles will reset related codes while driving.