Checking the air mass sensor: causes, diagnosis and troubleshooting

The air-mass sensor calculates the intake air mass in internal combustion engines. The engine control requires this variable in order to control a wide range of functions. This page explains the design and function of the air-mass sensor. You will also find information about how a faulty air-mass sensor can manifest itself, the causes leading to its failure, and how it can be checked in the workshop.

The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for suitably qualified personnel only.

Air-mass sensor function

The air-mass sensor is used to measure the intake air mass. It consists of a tubular housing with flow straightener, sensor protection, and sensor module. It is mounted in the intake pipe between the air filter housing and the intake manifold.

Two temperature-dependent metal film resistors affixed to a glass membrane are arranged in the air flow. The first resistor (RT) is a temperature sensor and measures the air temperature. The second resistor (RS) is used to detect the air flow rate. The resistor RS cools down to a greater or lesser degree depending on the intake air flow. In order to balance out the constant temperature difference between resistors RT and RS, the current flow through resistor RS must be dynamically regulated by the electronics. This heat flow is used as a measured variable for the respective air mass that is taken in by the engine. This measured value is required by the engine management control unit to calculate the fuel quantity required.

Testing the mass air flow sensor

The following test steps should be considered during troubleshooting:

Checking the plug connection

Check plug connection for tight fit and correct contact.

Checking the air-mass sensor

Check the air-mass sensor for damage.

Checking the measuring elements

Check the measuring elements for damage.

Checking the voltage supply

Check the voltage supply

Checking the connecting cables

Check the connecting cables between the removed control unit plug and the sensor plug for continuity (circuit diagram required for pin assignment). Reference value: approx. 0 ohms.

Electronic test of the air-mass sensor

Electronic test of the air-mass sensor through the engine management control unit. When a fault occurs, a fault code is stored in the control unit that can be read out using a diagnostic unit.