Checking air mass sensors

If it becomes apparent that the air mass sensor is faulty when reading out the diagnostic trouble code (OBD), then it should be checked before being replaced. Checking the voltage supply
The pin assignment of the sensor and the vehicle’s circuit diagrams should be observed during this process. The check can be carried out with a voltmeter, an oscilloscope or engine testers. It is also possible to connect the measuring instrument with the relevant test tips directly to the air mass sensor’s plug. 

Attention: When doing so, do not pierce the lines.

Checks when the ignition is turned on
When the ignition is on, there should be an on-board voltage of approx. 12 V between Pin 2 and the vehicle ground. There is a sensor voltage of approx. 5 V between Pin 3 and Pin 4. If these values are not reached, all affected lines and plugs must be checked for short circuits, interruption and transition resistances. 

Output voltage with still air
While the engine is stopped, the output voltage between Pin 3 and Pin 5 is approx. 1 V. Make sure to check that the measurement has not been distorted by a draught. 

Measuring under load
Start the engine! The measurement should be between 1.2 and 1.8 V here. During a burst of throttle up to maximum governed speed, 3.6 V to 4.4 V must be reached. If a measurement displays these values, then the air mass sensor is working. The fault must be looked for elsewhere, e.g. impurities in the intake air, contaminated or wrong air filters or leakages in the intake air system. Stuck intake manifold flaps, EGR valves or deposits on the throttle valve can also cause the signal from an intact air mass sensor to deviate from the calculated set-point value, causing an entry in the fault code memory.