Mass air flow sensor
The Mass Air Flow (MAF) sensor is also known as the air volume gauge or air mass gauge.
The MAF contains a hot wire sensor and an integrated temperature sensor. For a complete description refer to: http://en.wikipedia.org/wiki/Mass_flow_sensor
This is what a MAF sensor looks like:
The MAF sensor will foul over time, but the mixture formation can workaround it by adapting the mixture formation. When this adaptation gets out of range, we get the typical mixture codes - such as P0170/3. The degree of "adaption" can be seen using DAS.
The sensor itself does not have MB part number and must be purchased together with the housing A1120940048 (for M112) or A1130940048 (for M113). There are cheap 15 USD Chinese MAF sensors (e.g., sold on FENRIR parts store), but they have not proven to be reliable.
- M112: A1120940048 / BOSCH 0280217515 (100 EUR)
- M113: A1130940048 / BOSCH 0280217810 (115 EUR)
The Bosch MAFs have a cleaning cycle. During this cycle, the current is briefly increased to the hot wire sensor and this increases the temperature to something like 1000F. This is akin to a self cleaning oven.
To clean a MAF sensor, a special MAF sensor cleaner must be used, which leaves no deposit after drying out. A cleaner will remove oil and such deposits, but in the case of the Bosch MAF units, the deposit is probably more like a hard scale caused by deposits baking on.
The MAF sensor is held in place in the MAF housing with two Torx security screws which have a dimple in the center of the screws and hence a special driver is needed.
The following video shows how to replace the MAF housing: