Air Quality Sensor -G238-, Function

• There are different versions of the Air Quality Sensor -G238- -2-. This vehicle has an Air Quality Sensor -G238- that determines the NOx percentage (nitrogen) and CO percentage (carbon monoxide) of the air flowing by via a sensor. In addition, the Air Quality Sensor -G238- installed on this vehicle exchanges information with the Vehicle Electrical System Control Module -J519- via a local Data Bus. Therefore, note the exact allocation of the Air Quality Sensor -G238-. Refer to the Parts Information.
• The Vehicle Electrical System Control Module -J519- evaluates the measured value from the Air Quality Sensor -G238- -1- and transmits it to the Climatronic Control Module -J255- display and control head via the Data Bus. The Climatronic Control Module -J255- display and control head switches to recirculating air mode when requested, if there is no Shut-Off condition present using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.
• The Air Quality Sensor -G238- requires approximately two minutes 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.
• During the warm-up phase, the Air Quality Sensor -G238- malfunctions may occur under certain ambient influences. Only replace the Air Quality Sensor -G238- if the Air Quality Sensor -G238- is registered continually as faulty. If the malfunction indicator for the Air Quality Sensor -G238- changes from "static" to "sporadic" after turning the ignition off and then on again (waiting at least two minutes each time), then the Air Quality Sensor -G238- is OK and it is no longer necessary to observe the DTC memory using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.
• The Air Quality Sensor -G238- recognizes the pollutants in the surrounding air (mostly gasoline and/or diesel exhaust gases) and prompts the Climatronic Control Module -J255- display and control head to switch the A/C system to recirculating air mode.
• The Climatronic Control Module -J255- display and control head recognizes the type and intensity of the air pollutant on the signal from the sensor. The Climatronic Control Module -J255- display and control head then makes the decision depending on the exterior temperature and the intensity of the air pollutant using the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.

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

-- If the exterior temperature goes below -1 °C (30 °F) and in "ECON mode" (the A/C compressor is switched off and the indicator lamp in the AC  or A/C  button is off) the switch-over takes place first when there is a sharp increase in the pollutant concentration and only for approximately 15 seconds. If there is another increase in the pollutant concentration within approximately two minutes, then the Climatronic Control Module -J255- display and control head does not switch into the "automatic recirculating air mode".

If the A/C compressor is switched off (for example, "ECON" is on and the indicator lamp in the AC  or A/C  button is off), the Climatronic Control Module -J255-) display and control head limits the maximum duration of the "automatic recirculating air mode" to approximately 15 seconds (so that the windows do not fog up).

-- If the concentration decreases, the A/C system switches to the fresh air mode.

-- The duration of the "automatic recirculating air mode" depends on the version of the Climatronic Control Module -J255- display and control head. If the outside air temperature is below 10 °C (50 °F), then the system will stay on for approximately 30 seconds in "automatic recirculating air". After this, it switches back to fresh air mode. If, within approximately two minutes, an increase in the pollutant concentration occurs again, the Climatronic Control Module -J255- display and control head does not switch to "automatic recirculating air 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 (36 °F). If the temperature is below -5 °C (23 °F), then it is not possible to use the A/C compressor.
• At setting "automatic recirculating air mode" in the MMI (Multi Media Interface), the A/C compressor can also be switched on when the recirculating air mode is manually switched on (using the "recirculation button") up to a temperature of approximately - 5°C (23 °F).
• To ensure that the A/C system does not operate constantly in the recirculating air mode in areas with a consistently higher pollution, the Air Quality Sensor -G238- is self-learning (it adjusts its sensitivity to the environmental conditions).
• If outside air pollution load remains high for longer period of time, the Air Quality Sensor -G238- starts to adapt itself to the changing environmental condition through an adaptation program (learning program), 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 door can be closed. For this reason, on vehicles equipped with an Air Quality Sensor -G238- 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.
• In order to avoid frequent switching of the back pressure/fresh air door and the air recirculation door, a slight increase of pollutants in the surrounding air will not allow for immediate switching (the Air Quality Sensor -G238- does not send a request to the Climatronic Control Module -J255- display and control head). The filtering of the activated charcoal insert in the dust and pollen filter will be sufficient.
• In order to avoid frequent switching of the back pressure/fresh air door and the air recirculation doors, the request from the Air Quality Sensor -G238- for "Automatic recirculating air mode" remains for at least 25 seconds (minimum duration period), even if the pollutant concentration in the air is so far reduced that the recirculating air mode is no longer necessary.
• So that the fogged windows are cleared as quickly as possible, the Climatronic Control Module -J255- display and control head does not permit the recirculating air mode when "Defrost" mode is on (a request from the Air Quality Sensor -G238- is not processed).
• The Air Quality Sensor -G238- requires approximately 30 seconds to become operational once the ignition has been switched on (warm-up time). During this time there is no request from the Air Quality Sensor -G238- to the Climatronic Control Module -J255- display and control head for "automatic recirculating air mode".
• The Air Quality Sensor -G238- contains a highly sensitive electronic component which can be damaged if it comes in direct contact with solvents, fuels or chemicals. 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 -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 a minimum of two minutes.

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

-- Read the DTC memory of the Vehicle Electrical System Control Module -J519-. If the Air Quality Sensor -G238- 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 -G238- is no longer displayed as a malfunction or the malfunction was sporadic, then the Air Quality Sensor -G238- is OK (and no longer needs to be observed). Erase the DTC memory.