Air Quality Sensor -G238- Function

• The Air Quality Sensor -G238- uses a sensor to determine the NOx level (nitrogen oxide) and CO level (carbon monoxide) in the air flowing by and exchanges this information with the Vehicle Electrical System Control Module -J519- via the Data Bus.
• The measured value for the Air Quality Sensor -G238- is evaluated by the Vehicle Electrical System Control Module -J519- and is transmitted via the Data Bus to the Climatronic Control Module -J255- control head. Upon request, the Climatronic Control Module -J255- switches to recirculated-air mode if no shutoff condition is present. Use the :Vehicle Diagnostic Tester  in the "Guided Fault Finding"  function.

• The Air Quality Sensor -G238- requires a short period of time to reach its operating temperature after the ignition is switched on. During the warm-up phase, the function of the Air Quality Sensor -G238- is limited.
• The Air Quality Sensor -G238- detects pollutants in the surrounding air (usually gasoline and/or diesel exhaust) and causes the Climatronic Control Module -J255- to switch the A/C system to recirculating air mode.
• The Climatronic Control Module -J255- control head recognizes by the signal the type and intensity of the air pollutant on the signal from the Air Quality Sensor -G238-. Moreover, the Climatronic Control Module -J255- receives the sensitivity set by the Multimedia System Control Head -E380- via the Data Bus. Based on this information, the Climatronic Control Module -J255- then determines if and when the A/C system switches to recirculating air mode.

Example:

-- At an outside temperature above approximately 2 °C (35.6 °F), the recirculated-air mode is switched on upon request from the Air Quality Sensor -G238- when there is only a slight increase in the pollutants in the ambient air.

-- When the exterior temperature is less than approximately -1 °C (30.2 °F), and the A/C compressor is switched off, the change over only occurs when there is an increase in the pollutant concentration and for approximately 15 seconds. If another increase in the pollutant concentration occurs within approximately two minutes, the Climatronic Control Module -J255- does not switch over to "automatic recirculation mode" during this time.

• If the indicator lamp in the A/C  button does not illuminate, the Climatronic Control Module -J255- limits the maximum duration of the "automatic air recirculation" to approximately 15 seconds so that the windows in the interior do not fog up.

-- If the pollution concentration in the fresh air that is drawn in drops, the A/C system switches over to the fresh air mode.

• The duration of the "automatic air recirculation" depends on the Climatronic Control Module -J255- version, the setting on the Multimedia System Control Head -E380- and the Humidity Sensor -G355- measured values.

-- If the outside air temperature is lower than 10 °C (50 °F) and the maximum sensitivity is set on the Multimedia System Control Head -E380-, the system stays, for example, approximately 30 seconds in "automatic air recirculation". After this, it switches back to fresh air mode. If another increase in the pollutant concentration occurs within approximately two minutes, the Climatronic Control Module -J255- does not switch over to "automatic recirculation mode" during this time.

• "Automatic recirculating air 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 (35.6 °F). If the temperature is below -5 °C (23 °F), then it is not possible to use the A/C compressor.
• When the "automatic recirculating air mode" is set in the MMI (Multi Media Interface), the A/C compressor can be turned on up to a temperature of approximately -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 -G238- 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 -G238- 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 A/C system can operate over a longer period of time in recirculating 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 back pressure/fresh air door can be closed (and the air recirculation door is opened). For this reason, 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 -G238- does not send a request to the Climatronic Control Module -J255-). The filtering of the activated 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 -G238- 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 -J255- does not allow recirculation mode in the "defrost" mode (a request from the Air Quality Sensor -G238- is not implemented).
• The Air Quality Sensor -G238- requires a short period of time to become operational once the ignition has been switched on (warm-up time). During this time, no request is sent to the Climatronic Control Module -J255- control head for the "automatic recirculation mode".
• 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 -G238- is stored in the Vehicle Electrical System Control Module -J519- during the Air Quality Sensor -G238- warm-up time, even though the sensor is OK, proceed as follows:

-- Switch off the ignition for at least two minutes.

-- Switch on the ignition and wait at least two minutes.

-- Check the event memory of the Vehicle Electrical System Control Module -J519-. If the Air Quality Sensor -G238- still displays a static malfunction, perform Guided Fault Finding. Use the :Vehicle Diagnostic Tester  in the "Guided Fault Finding"  function. If the Air Quality Sensor -G238- is no longer displayed as a malfunction, or the malfunction is sporadic, the Air Quality Sensor -G238- is OK (and does not need to be observed). Erase the event memory.