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Hot Film Mass 6 (HFM6) Air Flow Sensors

Design - Function - Testing

In this technical article, we will cover the design, function and how to test and diagnose issues with Hot Film Mass 6 Air Flow Sensors, also known as HFM6 sensors. This is the next generation in hot film air mass sensors.


For precise control of the air fuel ratio, it is key for the air mass entering the engine to be measured correctly. To do this, hot film mass air flow sensors measure a small portion of the air entering the engine. The sensor takes into account positive and negative intake pulses caused by intake and exhaust valves opening and closing. Ambient pressure changes have no effect on air mass measurement.

HFM6 sensor housing and measuring tube are designed to accommodate engine measuring volumes from 370 to 970 kg/h. The tube is designed in a way to ensure airflow is uniform and to prevent against contamination directly upstream of the sensor element. HFM6 sensors use a two-section measuring tube. The sensor section uses a sharp curve for air to flow around. Debris or water droplets are unable to follow this curve and are separated from measured air flow. The contaminants exit the sensor through the second section. This results in fewer contaminants reaching the sensor element and increasing sensor life and reliability. The measuring tube is installed in an intake duct close to but downstream of the intake air filter.

Within the measuring tube is the measuring cell and the integrated evaluation electronics. The HFM6 measuring cell consists of a semiconductor substrate with a sensitive diaphragm surface with incorporated temperature sensitive resistors.

The HFM6 sensor is a thermal sensor with a centrally located heating resistor on the measuring cell that heats a sensor diaphragm and maintains it at a constant temperature. Temperature drops across each side of the controlled heating zone (on each side of the sensor). As air flows over the measuring sensor, the uniform temperature profile at the sensor changes. On the inlet side, temperature drops faster since the air flowing past this area cools it off. On the opposite side of the sensor, the temperature drops slightly, because the incoming air has been heated by the heater element. The change in temperature distribution leads to a temperature difference between the two measuring points. The voltage signal generated during this process in converted into a digital signal for further processing by the engine control module. The HFM6 also takes into account intake air temperature when determining air-mass. This greatly increases air-mass measurement accuracy.

Intake air temperature is measured by a thermistor (temperature-dependent resistor). Voltage drop through this thermistor passes through an analog-digital converter to create a digital signal representing intake air temperature. The engine control module uses this digital signal along with the air-mass digital signal to calculate an air mass correction using a stored map.

Testing HFM6 Sensors

When testing a mass air flow sensor, wide open throttle (WOT) snap the or perform a WOT test drive (accelerate from a stop and accelerate WOT until max RPM is reached. Usually by the second gear shift). Do not perform this test during unsafe or on roadways with other vehicles. Do not exceed posted speed limits. You will want to monitor the sensor signal under all RPM ranges of the test cycle.

You can verify sensor signal using a scan tool. The chart below shows kg/h in relation to KHz. This is for a 3.4 liter turbocharged engine. Khz varies depending on engine and size, but the important this to keep in mind here is the sensor output curve remains the way we expect it to look. You can still calculate engine air-mass volume and so a little math to get to the KHz reading. However it is easier to test the output for smooth operation. 

Both the signal and the correction signal can set fault codes and are monitored for circuit integrity and plausibility.

Testing with a Scan Tool

If available, use the graphing function of your scan to monitor sensor data over time. You can also few standard PID values. The vehicle shown below is an R56 MINI Cooper S model, known good PID value at idle. On the same MINI engine, expect to see 22 kg/h 2500 RPM in PARK no load and about 90 kg/h steady cruise at 2500 RPM.

Testing with a DVOM

Of course you can use a digital volt ohm meter (DVOM), however the reading displayed will be too slow to determine if there is a signal dropout or spike. The wiring diagram and RPM to Hz chart are once again from a turbocharged R56 MINI. As RPM increases so does frequency on the signal. The correction signal frequency will only fluctuate with ambient air temperature changes.

Testing with a Scope

You can monitor the pulse-train voltage output using  a scope. Once you have confirmed the signal it is tough to locate a glitch this way, but can be done if your scope has a deep record function. This function will allow you to record unlimited amounts of data during test drives or testing, then review and replay as needed.

When testing the correction signal (the following two patterns) don't expect much of a change in frequency if ambient air temp remains the same. Pattern one is recorded with the Key On and Engine OFF. Pattern two was recorded at Idle with a hot engine.

Correction Signal Key On Engine Off

Correction Signal Key On Engine Running (hot idle)

When testing the signal (the following two patterns) expect to see frequency increase as air-mass increase (airflow). Pattern one is recorded with the Key On and Engine OFF. Pattern two was recorded at Idle with a hot engine.

Signal Key On Engine Off

Signal Key On Engine Off

Testing with a Graphing Multi-Meter

The most efficient way to test this signal is using a graphing multi-meter. With signal frequency displayed over time (graphed) which gives you a look at frequency increase, decrease and trends. 

The first pattern below shows a 2.0 liter engine at idle. Frequency is smooth and reflects engine air-mass intake.

Perform a snap throttle test and monitor frequency increase, and the pattern looks similar to how we tested analog signals. You will also be able to see dropouts in the signal easier. The image below shows a 2.0 liter engine signal line with a snap throttle, then shortly after at idle the signal drops out (faulty sensor), shown by the two spikes in frequency. 

Now keep in mind when testing, the engine control module on HFM6 equipped vehicles take both digital signals (signal and correction signal) to calculate air-mass volume using a programmed map. If the engine control module (ECM or DME) is faulty, the calculation could be wrong. 

Need help diagnosing a computer control input or output on any make and model vehicle? Need some help setting up your scope or meter for tests like these? Give a call and we will be glad to help.