BMW E39 M5 Dual Mass Air Flow MAF Testing Specifications

Use the following information as an aide when testing BMW E39 dual MAF sensors. Keep in mind, a problem with mechanical engine components can affect MAF readings and besure you have a solid base engine before condeming parts.

KOEO (Key On – Engine OFF)taken at sensor back probing with DVOM
B1 AFM 1.02 Volts
B2 AFM 1.02 Volts
Using scan toolBoth AFM sensors read 0.9 Volt on GT1 under maf data stream.

Hot, engine running at idle taken at sensor back probing with DVOM
B1 AFM 1.30 Volts
B2 AFM 1.29 Volts
Using scan toolBoth AFM sensors read 1.0 Volt on GT1 under maf data stream.

2500 RPM taken at sensor back probing with DVOM
B1 AFM 1.85 Volts
B2 AFM 1.75 Volts
Using scan toolBoth AFM sensors read 1.5 Volt on GT1 under AFM data stream


IAT sensor not used on bank 2 AFM. However sensors are interchangeable.

Testing using a labscope. In Park or
Neutral, parking brake on.
Snap throttle to reach max voltage output.
(scope pattern shows a dual trace of both
The sensors should be within 0.1 volts of each other.

2000 Toyota Tundra 3.4 P0171

I arrived to a 2000 Toyota Tundra 3.4 running rough with a P0171 that was cleared by the shop. They had in hand fuel fouled spark plugs and no explanation as to why.

I connected my scan tool and observed the fuel control and feedback PIDs. I found fuel trim to be positive (indicating a lean condition) at idle and at 2500 RPM. This ruled out a vacuum leak.

The engine air flow was 7.56 grams per second (GPS) at idle. With an engine size of 3.4 lites I expected to see a GPS number close to that. A general rule of thumb: An engine flows about 1 GPS at idle per liter. For example a 2.0 liter engine would flow about 2 GPS at idle.


With a reading twice what it should be normally I wanted a close look at the sensor. Gaining access was easy, the mass air flow sensor is mounted in the air intake after the air filter near air box (arrow).


Once removed the mass air flow (MAF) hot wire was clearly covered in debris. This can lead to a skewed reading. When the hot wire is covered in dirt at idle (low air flow) the dirt acts like a radiator, cooling the wire and reporting a higher than actual reading. Off idle, the dirt acts like an insulator, protecting the hot wire element from the passing air and it’s cooling effect. This leads to a lower than actual air flow calculation.

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The dirt was cleaned and the MAF sensor was installed.


The reading dropped a few tenths of a volt but not quite enough.

After confirming the reference voltage and ground was good to the MAF sensor I suggested replacing it. The MAF sensor itself was faulty.