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Air Quality Sensor (G238), Checking






Air Quality Sensor (G238), Checking

Special Tools and Equipment

• Vehicle Diagnostic Tester with diagnostic cable (VAS 5051/5A)

• Commercial gas lighter

• In this vehicle the air quality sensor- C - and the Humidity sensor in fresh air intake duct (G657) - D - are combined in one component - A -.

• The air quality sensor - A - needs about 2 minutes to reach its operating temperature after the ignition is switched on. During the warm-up phase, the function of the Air quality sensor is limited.






• During the warm-up phase, air quality sensor malfunctions may occur under certain ambient conditions. Replace the air quality sensor Air quality sensor only if it is registered continually as faulty. If the malfunction display for the Air quality sensor changes after switching off the ignition and then switching it back on (wait at least 2 minutes) from "static" to "sporadic", the air quality sensor is OK and it is no longer necessary to pay attention to the DTC memory entry (erase the event memory) using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.

• Certain A/C system functions (for example control of automatic recirculation mode) can be switched on and off via the MMI (Multi Media Interface) under the "A/C system" function in the "car"/"car systems" menu. The A/C system control can also be influenced via presettings in the MMI (Multi Media Interface) under the "A/C system" function in the "car"/"car systems" menu. Therefore, first check the presetting in the MMI in the case of a problem with these components. Refer to Infotainment/MMI operating instructions.

Test Prerequisites

• Vehicle standing in a clean atmosphere (away from running engines, exhaust vents, etc.)

• Engine compartment and the plenum chamber are clean (not contaminated with oil or fuel)

• The engine compartment and the plenum chamber were not sprayed with cleaning agents or preservatives containing solvents

• Automatic recirculating air mode is switched on.

• It is possible to switch the automatic recirculation mode on or off via the "A/C" function in the "car"/"car systems" menu in the MMI (Multi Media Interface). Refer to Infotainment/MMI operating instructions.

Check the function

- Remove plenum chamber cover. Refer to - A -






• To prevent water from running via intake shaft - C - into the A/C unit when plenum chamber cover - A - is installed, plenum chamber cover - A - must not be damaged. Moreover, plenum chamber cover - A - must be correctly and completely engaged in frame - D - of windshield - B -.

- Connect the vehicle diagnostic tester to the diagnostic connection of the vehicle with the ignition switched off and initiate Guided Fault Finding using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.

- Start engine.

- Check the event memory of the Climatronic control module (J255) using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.

- Set Auto mode on the Climatronic control module.

- Set automatic recirculation mode via the MMI (Multi Media Interface) in the Climatronic control module. Refer to Infotainment/MMI operating instructions.

- wait 2 minutes. (Warm-up time of the air quality sensor).

- Select the read measured values function via the vehicle diagnostic tester and read out the measured values of the air quality sensor using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.

• The air pollution and the NOx level (nitrogen oxide) and CO level (carbon monoxide) in the air measured by the air quality sensor is displayed in the read measured values function; explanation of possible displays using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.

- Rotate the air quality sensor - A - approximately 90 °C (194 °F) and remove it from the fresh air intake - E -.






- Allow a little bit of gas from the lighter - B - to flow past from the top area - A - of the air quality sensor.






• If the air quality sensor comes in contact with cigarette smoke or with gas from a lighter, it will react since butane gas is heavier than air; therefore, remove the air quality sensor.

- In the read measured values function of the Guided Fault Finding, select and read out the measured values of the air quality sensor using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.

Specified Values

• In the display fields, the measured values for the CO level change with respect to the initial value, thus indicating that the CO level sensor in the air quality sensor is functioning properly and the air quality sensor electronics are evaluating the measured values correctly.

• The NOx level sensor currently cannot be tested with workshop equipment. However, if the air composition around the sensor changes, the displayed value for the NOx level also changes.

- Wait a short period time (approximately 1 minute, depending on amount of test gas flowing into the sensor).

- Read the display fields with the air quality sensor measured values.

Specified Values

• The measured values will return to the output values and by doing so, show that both sensors are working correctly, and that the sensor electronics evaluated the measured values correctly.

• Due to changed pollution values, it is possible that different values can be displayed than those by the first measuring.

- Read the display with the measured values for the A/C operating status (recirculation/fresh air operation).

Specified Values

• The display field will show that the A/C is working in the fresh air mode.

• If a value deviating from the fresh air mode is displayed in the read measured values function of the Guided Fault Finding, there is already a request for recirculation mode (for example, due to the request for maximum cooling output or the manual recirculation mode setting). Select an operating mode that does not require recirculation mode.

• If it is displayed in the read measured values function of the Guided Fault Finding that the air quality sensor is already requesting automatic recirculation mode, clean the plenum chamber or move the vehicle into an area where the air is cleaner. Check using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.

- Allow a little bit of gas from a lighter - B - to flow past from the top area - A - of the air quality sensor.






- Read the recirculation/fresh air mode display.

Specified Values

• The read measured values function of the Guided Fault Finding displays that a request for recirculation mode is present and the A/C system is working in recirculation mode.

• If the read measured values function of the Guided Fault Finding displays that the air quality sensor is requesting automatic recirculation mode but this cannot be implemented due a shut-off condition of the Climatronic control module, eliminate the cause of this shut-off condition using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.

• In order to avoid frequent shifting of the recirculated air/fresh air doors and the recirculation doors, the requirement of the air quality sensor for Automatic air recirculation mode remains for at least 25 seconds (minimum duration period), even if the pollutant concentration is so far reduced that recirculated air mode is no longer necessary.

- Wait a short period time (approximately 1 minute, depending on amount of test gas flowing into the sensor).

• The read measured values function of the Guided Fault Finding displays that a request for recirculation mode is no longer present and the A/C system is working again in fresh air mode.

If the function of air quality sensor is OK for this test and there is a customer complaint continue as follows:

- Check the dust and pollen filter for soiling, refer to => [ Dust and Pollen Filter ] Service and Repair.

- Connect a vehicle diagnostic tester to the diagnostic connection of the vehicle with the ignition switched off and initiate Guided Fault Finding using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.

- Via the vehicle diagnostic tester, select the read measured values function using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.

• Second person is required to read out values displayed whilst driving.

Observe the safety instructions using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function

- Read out the displays in the read measured values function of the Guided Fault Finding for the measured values of the air quality sensor and the recirculation/fresh air mode request.

- Drive the vehicle into an area with a relatively clean atmosphere (system in fresh-air mode).

- Then drive vehicle into an area where the atmosphere is polluted (for example onto an uphill road with lorry traffic).

- Read the displays in the different display fields. The displays in the different display fields must change; for example, when driving through a Diesel exhaust cloud.

• Also read and heed descriptions => [ Dust and Pollen Filter ] Service and Repair, => [ Air Quality Sensor (G238) ] Description and Operation and => [ Humidity Sensor In Fresh Air Intake Duct (G657) ] Humidity Sensor In Fresh Air Intake Duct (G657).

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