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
In addition to the Humidity Sensor In Fresh Air Intake Duct -G657- -1-, an Air Quality Sensor -G238- -3- is integrated in the housing -2- for an Automatic climate control.
There are different versions of the Air Quality Sensor -G238- -3- and the Humidity Sensor In Fresh Air Intake Duct -G657- -1-. Pay attention to the correct allocation. Refer to the Parts Information.
- 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.
Certain A/C system functions (for example, control of "automatic recirculated-air 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 presetting 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.
- From the measured values, the Climatronic Control Module -J255- calculates the current, relative humidity of the drawn-in fresh air and the moisture in the passenger compartment from the Humidity Sensor -G355- measured values. 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 recirculation mode", by preventing engine shutoff via the start-stop function, etc.
Check the Humidity Sensor In Fresh Air Intake Duct -G657-. Use the : Vehicle Diagnostic Tester in the "Guided Fault Finding" function and Wiring diagrams, Troubleshooting & Component locations.
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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.