Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Humidity Sensor In Fresh Air Intake Duct -G657- Function
There are different versions depending on the vehicle date of manufacture and the A/C system version ("Basis" or "Convenience") is installed. The "Basis" A/C system does not have a humidity sensor in the Humidity Sensor in Fresh Air intake Duct -G657- /Air Quality Sensor -G238- until the MY 2012 changeover. From MY 2012 it has a humidity sensor in Humidity Sensor in Fresh Air intake Duct -G657- without an Air Quality Sensor -G238- (black housing). From MY 2012, the "Convenience" A/C system has an Air Quality Sensor -G238- without a Humidity Sensor in Fresh Air intake Duct -G657-. From this change, a Humidity Sensor in Fresh Air intake Duct -G657- is installed with an Air Quality Sensor -G238- (brown housing).
Only vehicles with the "Convenience" A/C system have an Air Quality Sensor -G238- without a Humidity Sensor in Fresh Air intake Duct -G657- (from start of production up the changeover) Refer to the Parts Information.
From MY 2012, the vehicle version was changed-over as a running change beginning with Audi A4 and Audi A5. Refer to DISPLAY AND CONTROL HEAD LOCATION . Vehicles, built before the changeover and with the "Convenience" A/C system, have an Air Quality Sensor -G238- (not installed on the "Basis" version). For vehicles built since this changeover (and for the Audi Q5 Hybrid), a Humidity Sensor in Fresh Air intake Duct -G657- is installed on the "Basis" version and a Humidity Sensor in Fresh Air intake Duct -G657- with the Air Quality Sensor -G238- is installed on vehicles with the "Convenience" version.
This is how to tell the difference: the Humidity Sensor In Fresh Air Intake Duct -G657- with an Air Quality Sensor -G238- has a brown housing (running change from MY 12). For the Humidity Sensor in Fresh Air intake Duct -G657- version without the Air Quality Sensor -G238-, the housing is black (running change from MY 2012). For the Air Quality Sensor -G238- version without a Humidity Sensor in Fresh Air intake Duct -G657-, the housing is black and the Humidity Sensor in Fresh Air intake Duct -G657- -3- is not present (gradual discontinuation from MY 2012).
- The Humidity Sensor in Fresh Air intake Duct -G657- -3- with the Air Quality Sensor -G238- -1- is installed in the same housing -2- (housing color "brown") on vehicles with the "Convenience" A/C system.
- If the vehicle has the "Basis" A/C system, then only the Humidity Sensor in Fresh Air intake Duct -G657- -3- is present. The Air Quality Sensor -G238- -1- is not installed in the housing -2- (housing color "black").
- The Humidity Sensor In The Fresh Air Intake Duct -G657- is a highly sensitive electronic component that can be damaged if it comes in direct contact with fluids, solvents, fuels and certain chemical compounds.
- The function of the Humidity Sensor In The Fresh Air Intake Duct -G657- 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.
- The Humidity Sensor In The Fresh Air Intake Duct -G657- determines the temperature and the moisture in the air flowing by and exchanges the information with the Vehicle Electrical System Control Module -J519- via the Data Bus.
- The measured value for the Humidity Sensor In The Fresh Air Intake Duct -G657- 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.
- The Climatronic Control Module -J255- uses the measured values to calculate the current relative humidity of the drawn-in fresh air. The calculated humidity value is used to detect whether there is a risk of the windshield, rear window, and door windows fogging up on the inside. If that is the case, the Climatronic Control Module -J255- changes various characteristic curves to counteract this. This can be performed, for example, by lowering the evaporator temperature (by activating the A/C compressor), by raising the response thresholds for the request for "automatic recirculating air mode", by preventing engine Shut-Off via the start-stop function, etc.
- The Humidity Sensor in Fresh Air intake Duct -G657- needs about two minutes until it has reached its operating temperature after the ignition is switched on. During the warm-up phase, the function of the Humidity Sensor in Fresh Air intake Duct -G657- is limited.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.