Pierburg MAF Sensor: Fixing Low Readings (Articles)

Basic information about LMM can be found under https://community.dieselschrauber.org/en/viewtopic.php?t=3347.

Before making any circuit adjustments to the mass airflow sensor (MAF) voltage, the MAF sensor should be checked for deposits. Clean the protective grille on the air filter side with a tissue (are there significant dirt traces?). If necessary, clean the sensor. Then, during a test drive, verify the mass airflow readings.
If the values are correct after cleaning, then the soldering repair described here is obviously unnecessary.

Furthermore, all of this only makes sense if the turbocharger boost pressure is correct and the rest of the engine is also in perfect condition (no breathing problems due to EGR sludge, no debris blocking the catalytic converter in the exhaust, etc.).


To repair the mass airflow sensor, you must first remove its cover.
Using a thin screwdriver or similar tool, open the latch closest to the first point where you need to pry. Simultaneously, use a wide screwdriver to lever the lid upwards.
In this process, the sealant in the groove slowly tears, causing the lid to open reluctantly.
When lifting the lid further, open the other locking points. Always proceed gently to avoid damaging the lid!
Intermittently, apply the flat-head screwdriver to the opposite side of the lid to open the remaining adhesive edges.

The LMM utilizes an internal Wheatstone bridge, with a PTC or a hot-film element located in its branches. By soldering in a resistor, as shown in the appendix, the circuit is biased towards higher LMM signal voltages.

According to VCDS (VAG-COM), a resistance of just 22 kΩ causes an increase of approximately 100 mg/stroke across the entire measurement range. Smaller resistances cause a greater increase in the measured air mass values.

To avoid damage caused by static discharge, it is recommended to ground the soldering iron and the person performing the soldering work to the ground connection of the laser marking machine during soldering operations.

Then, use VCDS (VAG-COM) to check the effect on the air mass measurement values.
When the engine is idling, the EGR system should be deactivated, otherwise the EGR control will distort the readings. For the 1.9-liter engines, an optimal value is around 500 mg per stroke.
For the upper measurement range, perform a full-throttle test in 3rd gear, driving from approximately 1500 to 4000 rpm, and compare the actual and target air mass values (it's best to log the data).
VCDS (VAG-COM) constantly displays the required minimum value in the target field, at which point the fuel injection quantity has not yet been reduced. The final additional resistor should be selected such that this target value is never exceeded, even under full boost pressure.

If the LMM (lambda measuring module) reading is increased further, the engine may produce more soot when accelerating sharply from partial load, which could, among other things, jeopardize passing the next emissions test.

For the final reassembly of the LMM cover, apply a thin layer of silicone sealant or similar material into the groove.