Mass Airflow sensor (MAF) is essential for the good running of a modern engine. See the Wikipedia article for an introduction to the theory, a university lab procedure for details on the science behind, or a specifications sheet for examples of electronic diagrams and signal charts. I did not find documentation for the exact type of sensor I’ll describe later.
MAF are sensitive to contamination from particles and fumes bypassing the air filter. This leads to driveability problems like hesitation, reduced power, and poor fuel economy, and might also cause lighting the “check engine” marker.
New, non-OEM replacement units are offered for sale on internet (Ebay and elsewhere) for very low prices, but I found only little information regarding their reliability. For example one forum contributor mentions that such units last only few months after which they fail. A gentleman from USA describes how he replaced a defective MAF on his Mercedes Benz with a non-oem unit he ordered from China. The new sensor cleared the ECU error codes, and appeared to be good in several tests, but it did not solved the drivability problems, which lead to new and expensive repair sessions. In the end, after one year, the author decided to purchase a genuine unit, which immediately restored the engine’s characteristics. This suggests that some cheap sensors might generate apparently correct signal (voltage, frequency, etc. are in range), but won’t measure the actual air flow. When diagnosing an engine, this is even worse than a totally dead sensor.
I decided to test a non-oem MAF on my Opel Astra (2001, 1.8 16V, Z18XE) and to post the results on the Internet. The methodology is very simple: I buy one unit from Ebay, I install it, replacing the original, working, one, and I see if it’s working, and for how long. I don’t have an oscilloscope or OBD2 scanner, so I can’t show any quantitative data.
Here is the original MAF, from the car. The sensing element is of “hot film” type. Manufacturer’s name (SIEMENS), part number (5WK9 606) and GM code (90 530 463) are embossed on the body.
The intake side is protected with a plastic grille and a wire mesh.
The outlet side has a removable plastic grille, secured by plastic clips. If needed, it can be removed using two fine screwdrivers.
Inside the barrel there are just three components: a thermistor (which is the intake air temperature sensor), and two thin glass-like blades soldered to a metal holder. Those are the actual hot-film elements. In my case they look pretty clean, so I won’t touch them.
The new unit:
Externally the new MAF looks quite similar. Intake and output sides are protected with plastic caps.
Unlike the original, there is a code etched on the top side, and nothing on the bottom. No indication whatsoever of manufacturer or origin.
Inside we notice some significant differences: instead of the high-tech blades, there is a plain resistor and an unidentified part. I have no idea of what it is, I can only guess that it might be a cylinder shaped hot-film element.
I installed the new unit on the car without resetting the ECU. Surprisingly enough, the engine runs exactly as before, perhaps a little more powerful. During a short test drive I could not sense any misbehaviour. Therefore, I conclude that, out of the box, the non-oem sensor seems to be working - it’s not just a fake. This is not very relevant after such a short test period, so I’ll post new findings after I'll drive few hundreds/thousands kilometres.
Thanks for reading!