Switches & Sensors
Air Mass Sensor Function
Installed in every car for more than a decade, the air mass sensor is an essential component in the air supply system. Its signal is used for calculating fuel injection quantity and for controlling the exhaust gas recirculation on diesel engines.
Not an “Air Flow Meter”
Air mass sensors are sometimes misleadingly referred to as air flow meters. However, the air flow meter only detects air volume. As the air mass also takes the temperature and pressure into account, it is considerably more accurate.
Anatomy and Evolution
The complete air mass sensor consists of a duct (“tube”) in which the intake air streams past the sensor – the sensor measures the movement of this air over the filament. Depending on the application and vehicle, the air mass sensor is available either fully integrated in a plastic tube, or with sensor as an individual insert.
Older models were equipped with a hot wire sensor element. By briefly heating it after the engine was cut, the hot wire was “burnt free” of impurities. More recent models work with a filmlike heating resistor so the “burning-off” process is no longer necessary.
Defective air mass sensors will send faulty signals or no signal at all. Incorrect input signals can cause the ECU to actuate other components incorrectly. For this reason, error messages such as “mixture control too lean/too rich” or “EGR flow rate too high/too low” may also indicate a defective air mass sensor.
Common Causes of Air Mass Sensor Failure
Leaks in the intake air system allow dirt particles to enter and hit the sensor at high speed, destroying the sensor element.
Excessive oil mist from the crankcase ventilation can result in oil reaching the sensor. Splash water can reach the clean air side via the air filter and damage the sensor. Salt water, from road salt, intensifies this effect.
Oil particles from oil-wetted sports air filters and faults during servicing (e.g. fitment of an incorrect or inferior quality air filters) may cause damage to the air mass sensor.
Other causes include: defective exhaust gas recirculation or tank ventilation valves and damage to the turbocharger can also cause the air mass sensor to send a faulty signal.
Dirt or oil ingress on an air mass sensor can cause so-called “sporadic faults”. This is where the malfunction indicator lamp (MIL) suddenly lights up and then extinguishes itself. Measurements can be falsified by moisture, oil mist or dirt which is temporarily interpreted as a fault.
Remember: although a failure or incorrect function of the on-board diagnostic system (OBD) will be detected this is not necessarily the actual cause of the failure or fault. If the air mass sensor does appear to have failed, the cause of the failure often lies elsewhere. In fact, almost all air mass sensors submitted to Pierburg for warranty claims are full of oil or dirt, or look as if the unit has been “sandblasted”.
Pierburg has a strong heritage in air management and supplies air mass sensors as original equipment to a number of vehicle manufacturers, including Mercedes, Opel, Volkswagen and Peugeot. The Pierburg emission control range includes over 57 air mass sensor references.