Climate Control Exhaust Gas Sensor: Description and Operation
Air Quality Sensor (G238)
• The Air quality sensor/ Humidity sensor in fresh air intake duct (G657) is available in different versions. In this vehicle, a sensor - A - is installed in which the Air quality sensor - C - and the Humidity sensor in fresh air intake duct - D - are combined in a single component. Refer to the Electronic Parts Catalog.
• The Air quality sensor - C - uses a sensor to determine the NOx level (nitrogen oxide) and CO level (carbon monoxide) in the air flowing by. The air quality sensor installed in this vehicle exchanges information with the vehicle electrical system control module (J519) via the data bus. Therefore, ensure the proper version of the air quality sensor/humidity sensor in fresh air intake duct. Refer to Electronic Parts Catalog.
• The measured value of the Air quality sensor - C - is evaluated by the vehicle electrical system control module and is transmitted via the data bus to the Climatronic control module (J255). Upon request, the Climatronic control module switches to recirculation mode if no shut-off condition is present. Check 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.
• The air quality sensor - C - 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.
• The air quality sensor - C - detects pollutants in the ambient air (usually gasoline and/or diesel exhaust) and causes the Climatronic control module to switch the A/C system to recirculation mode.
The Climatronic control module, detects the type and intensity of air pollution on the basis of the signal from the air quality sensor. The Climatronic control module receives the sensitivity set by the multimedia system control head (E380) via the data bus. On the basis of this information, the Climatronic control module then decides whether and when the A/C system will be switched to recirculation mode. Refer to Infotainment/MMI operating instructions and the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.
Example:
- At an outside temperature above approximately +2°C (35.5 °F), the recirculation mode is switched on upon request from the air quality sensor when there is only a slight increase in the pollutants in the ambient air.
- If the outside air temperature goes below -1°C (30 °F) and ECON mode is activated (the A/C compressor is switched off and the indicator lamp in the AC button is off), the switch-over takes place first when there is a significant increase in the pollutant concentration and only for approximately 15 seconds. If there is another increase in the pollutant concentration within approximately 2 minutes, the Climatronic control module does not switch to recirculation mode in this time.
- If the A/C compressor is switched off (ECON mode is on and the indicator lamp in the AC button is off), the maximum duration of the automatic recirculation mode is limited to 15 seconds by the Climatronic control module (so that the windshield, rear window, and door windows do not fog up).
- If the concentration decreases, the air conditioner switches to the fresh air mode.
- The duration of automatic recirculation mode depends on the version of the Climatronic control module and the setting on the multimedia system control head. If the outside air temperature is below +10°C (50 °F) and the maximum sensitivity on the multimedia system control head is reached, the system will stay on for approximately 30 seconds in automatic recirculation mode. After this, it switches back to fresh air mode. If another increase in the pollutant concentration occurs within approximately 2 minutes, the Climatronic control module does not switch over to automatic recirculation mode during this time.
• Automatic air recirculation mode can be switched off at any time. If the function is active, the A/C compressor is switched on with a request for automatic recirculating air mode and also when the outside air temperature is below +2 °C (35.5 °F). If the temperature is below -5 °C (23 °F), then it is not possible to sue the A/C compressor.
• If automatic recirculation mode is selected in MMI (Multi Media Interface), then the A/C compressor can be switched on by manually switching on recirculating air mode (using the air recirculation button) down to -5 °C (23 °F).
• To ensure that the A/C system does not operate constantly in recirculation mode in areas with a consistently high pollution, the Air quality sensor is self-adapting (it adjusts its sensitivity to the environmental conditions).
• If the outside air pollution load remains relatively high for a long period of time, the Air quality sensor starts to adapt itself to the changing environmental conditions through an adaptation program (learning program) so that the air recirculation request in evenly polluted ambient air is therefore generally shorter than 12 minutes. If a succession of peaks in the polluted air is detected, the air conditioning system can operate over a longer period of time in recirculated air mode.
• A certain amount of time is required for repositioning of the air conditioning system doors. In the event that a sudden increase in pollutants is encountered (for example when driving through a diesel exhaust cloud) gaseous pollutants can be taken in with the fresh air into the passenger compartment until the door can be closed. For this reason, on vehicles equipped with a Air Quality Sensor a dust and pollen filter is installed with a charcoal layer. A filter that is saturated with pollutants is no longer effective and needs to be replaced.
• To prevent excessive operation of the back pressure/fresh air door and the air recirculation door, the door is not actuated immediately if there is a nominal increase in pollutants in the outside air (the air quality sensor does not send a request to the Climatronic control module). The filtering of the charcoal insert in the dust and pollen filter will be sufficient.
• In order to avoid excessive operation of the back pressure/fresh air door and the air recirculation door, a request from the air quality sensor for automatic recirculation mode remains active for at least 25 seconds (minimum duration period), even if the pollutant concentration is so far reduced that recirculation mode is no longer necessary.
• To defog fogged-up windshield, rear window, and door windows as quickly as possible, the Climatronic control module does not allow recirculation mode in the defrost mode (a request from the air quality sensor is not implemented).
• After the ignition is switched on, the Air Quality Sensor requires approximately 30 seconds to be operational (warm-up time). During this time there is no request from the air quality sensor to the Climatronic control module, for automatic recirculation mode.
• Sensor - A - (combination sensor with the air quality sensor/Humidity sensor in fresh air intake duct) is a highly sensitive electronic component that can be damaged if it comes in direct contact with fluids, solvents, fuels and certain chemical compounds. Moreover, the function of the Humidity sensor in fresh air intake duct can be severely affected by direct contact with water so that correct moisture values can no longer be measured (briefly or permanently depending on the composition of the fluid). For this reason, do not install sensors that may have come into contact with these substances.
• Under certain ambient influences, when switching on the ignition (for example, when the air pollution is high), if a fault on the air quality sensor is stored in the vehicle electrical system control module during the air quality sensor warm-up time, even though the sensor is OK, proceed as follows:
- Switch off the ignition for minimum 2 minutes.
- Switch on the ignition and wait at least 2 minutes.
- Read the event memory of the vehicle electrical system control module. If the air quality sensor still displays a static malfunction, perform Guided Fault Finding using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function. If the Air quality sensor is no longer displayed as a malfunction, or the malfunction is sporadic, the Air quality sensor is OK (and does not need to be observed). Erase the event memory.