Mini Hot Film Air Mass Sensor (HFM)

A compact mini hot film air mass sensor (HFM manufactured by Siemens) is used in the secondary air system for the M54 in the E60 (US market).

The mini HFM detects the air mass supplied by the secondary air pump. This function monitors the secondary air system for OBD compliance.

When the mini HFM detects no air mass or insufficient air mass, a fault is stored in the ECM and the Malfunction Indicator Light (MIL) is activated.

The mini HFM has a compact pipe shaped design with O-ring connections.

Fig 1: Identifying Mini Hot Film Air Mass Sensor (HFM)

Courtesy of BMW OF NORTH AMERICA, INC.

The "Malfunction Indicator Light" ( MIL) will be illuminated under the following conditions:

Upon the completion of the next consecutive driving cycle where the previously faulted system is monitored again and the emissions relevant fault is again present.
Immediately if a "Catalyst Damaging" fault occurs.

Fig 2: Identifying Malfunction Indicator Light

Courtesy of BMW OF NORTH AMERICA, INC.

The illumination of the light is performed in accordance with the Federal Test Procedure (FTP) which requires the lamp to be illuminated when:

A malfunction of a component that can affect the emission performance of the vehicle occurs and causes emissions to exceed the standards required by the (FTP).
Manufacturer-defined specifications are exceeded.
An implausible input signal is generated.
Catalyst deterioration causes HC-emissions to exceed the standard (FTP) limit.
Misfire faults occur.
A leak is detected in the evaporative system, or "purging" is defective.
ECM fails to enter closed-loop oxygen sensor control operation within a specified time interval.
Engine control or automatic transmission control enters a "limp home" operating mode.
Ignition is on (KL15) position before cranking = Bulb Check Function.

Within the BMW system the illumination of the Malfunction Indicator Light is performed in accordance with the regulations set forth in CARB and as demonstrated via the Federal Test Procedure (FTP).

The "Malfunction Indicator Light" can be diagnosed with an aftermarket Scan Tool that allows Technicians without BMW Special Tools or Equipment to Diagnose an emission system failure.

With the use of a universal scan tool, connected to the "OBD" DLC (located in the driver's side lower A-pillar area) an SAE standardized DTC can be obtained, along with the condition associated with the illumination of the "Malfunction Indicator Light".

Fig 3: Identifying OBD DLC

Courtesy of BMW OF NORTH AMERICA, INC.

Using the DISplus or GT1, a fault code and the conditions associated with its setting can be obtained prior to the illumination of the "Malfunction Indicator Light".

There is no direct connection to the OBD diagnostic connector. The ECM is connected to the SGM via the PT-CAN Bus. The OBD diagnostic connector is connected to the SGM via the Diagnosis Bus.

Fig 4: Identifying SGM Diagnostic Bus Components

Courtesy of BMW OF NORTH AMERICA, INC.

The hardwire shown above between the CAS2 and the PT-CAN control modules is the KL15 (w up) signal.