Humidity Sensor: Service and Repair

Air Quality Sensor (G238)/Humidity Sensor in Fresh Air Intake Duct (G657)

• There are different versions depending on the vehicle date of manufacture and which A/C system ("Basis" or "Comfort") is installed. Refer to => [ Air Quality Sensor (G238) and Humidity Sensor in Fresh Air Intake Duct (G657) Function ] Description and Operation.

• The air quality sensor is installed only on vehicles with the "Comfort" A/C system.

• The humidity sensor in fresh air intake duct is only installed on MY 12 vehicles as a running change. Refer to => [ Air Quality Sensor (G238) and Humidity Sensor in Fresh Air Intake Duct (G657) Function ] Description and Operation.

• The air quality sensor / humidity sensor in fresh air intake duct are highly sensitive electronic components that can get damaged if they come in direct contact with solvents, fuels or chemicals (e.g. impurities may enter through area - A -). Therefore, never install an air quality sensor / humidity sensor in fresh air intake duct that was stored, for example, in a tool box.






• Following removal, do not set down air quality sensor / humidity sensor in fresh air intake duct in areas where it could come into contact with solvents, fuels or certain chemical compounds (liquids or vapors).

• There are different versions of the air quality sensor / humidity sensor in fresh air intake duct with different functions. Refer to the Parts Catalog.


For vehicles without a high voltage system

- Switch off ignition.

- Pull the left rubber seal - B - as far as necessary.






- Remove the fresh air intake cover on the wind cowl - A - or the wind cowl (different styles).

• If necessary, carefully loosen the wind cowl from the windshield cross member - C -

All vehicles

- Turn the air quality sensor / humidity sensor in fresh air intake duct - A - approximately 90° to the left and remove it from the fresh air intake - C -.






- Disconnect the connector - B - from the air quality sensor / humidity sensor in fresh air intake duct.

Terminal assignment in connector - B -

1 B+ (terminal "15")
2 Ground (terminal "31")
3 Signal wire to the Vehicle Electrical System Control Module (J519).

• The signal from the air quality sensor / humidity sensor in fresh air intake duct cannot be monitored by workshop equipment.

• The vehicle electrical system control module evaluates the data from the air quality sensor / humidity sensor in fresh air intake duct and then transmits it to the Climatronic control module (J255) display and control unit via the Data bus.

• There are different versions of the air intake shroud for the fresh air intake - C - (with and without a mount for the for the air quality sensor / humidity sensor in fresh air intake duct - A -, in the front or on the side). Refer to the Parts Catalog.

• The measured values from the air quality sensor / humidity sensor in fresh air intake duct are displayed in the Climatronic control module display and control head measured values block and in the vehicle electrical system control module (J519) measured values block. Explanatory notes of the displays refer to the Vehicle Diagnostic Tester in "Guided Fault Finding" function.