2021-23 Envision & 2020-22 CT4 Windshield Fogging? Your Climate Control Sensor May Be to Blame

Symptoms of a Failing Humidity and Windshield Temperature Sensor

A faulty humidity sensor typically does not trigger a check engine light. Instead, it causes problems with the automatic climate control system's performance. You may notice one or more of the following issues:

• Persistent Windshield Fogging: The most common symptom is a windshield that fogs up easily, especially in damp or cool weather, and takes a long time to clear. The automatic system fails to prevent the condensation from forming in the first place.
• Erratic HVAC Behavior: The climate control may seem to have a mind of its own. The air conditioning might run on a cool day when it isn't needed, or the fan speed might change unpredictably as the system struggles to manage the incorrect data it's receiving.
• Ineffective Defrosting: When you manually turn on the defrost setting, it may seem less effective or you may have to leave it running constantly to keep the glass clear. This is because the automatic function that should be assisting is not working.
• AC Runs Constantly or Not at All: The sensor may send a false high-humidity signal, causing the AC compressor to run continuously to dry the air. Conversely, it might fail to detect rising humidity, preventing the AC from engaging when it's needed to prevent fog.

Known Issues, TSBs, and Recalls

As of this writing, there are no specific recalls or widespread, documented failures for this exact humidity sensor on the 2021-2023 Buick Envision or 2020-2022 Cadillac CT4. However, GM has addressed similar issues on other vehicles.

For example, Technical Service Bulletin 17-NA-201 was issued for other GM models that experienced excessive condensation on the windshield. The fix involved removing foam pads from the defrost door and reprogramming the HVAC control module. While this TSB does not apply to the Envision or CT4 models in this guide, it shows that fogging issues are on GM's radar and are often solved with a combination of minor physical changes and software updates. Similarly, forum discussions for the CT4 mention various AC issues, from refrigerant leaks due to rubbing hoses to faulty control modules, highlighting the complexity of the climate system.

Buying a Used Humidity and Windshield Temperature Sensor

A used OEM sensor is a cost-effective alternative to a new part, especially since affordable aftermarket options are not widely available for these specific models. When buying a recycled sensor, follow these guidelines to ensure you get a good part.

What to Inspect Before You Buy

• Verify the Part Number: The most reliable OEM part number is 84748631, which supersedes the earlier number 84591804. You may also see part number 13531872 used for these vehicles. Match the number on the part you are buying to one of these to ensure compatibility.
• Check the Sensor Lens: The part of the sensor that sits against the glass has a clear gel pack or lens. Inspect it for cloudiness, damage, or air bubbles. The mounting surface must be perfectly clear for an accurate reading.
• Examine the Connector: Look at the electrical connector pins. They should be straight, clean, and free of any corrosion or damage. Bent or corroded pins will prevent the sensor from communicating with the vehicle.
• Inspect the Housing: Check the plastic housing for any cracks or signs of damage. A cracked housing could allow moisture to enter and damage the internal electronics.
• Consider Mileage: Since this is an electronic component, choosing a sensor from a lower-mileage donor vehicle is always a good practice.

Cost Breakdown for Replacement

Replacing the humidity and windshield temperature sensor is relatively inexpensive, but the potential need for professional calibration can add to the cost.

Installation and Calibration

Physically replacing the sensor is straightforward, but completing the job correctly may require a professional tool.

Difficulty: 2 out of 5 (1 for physical install, +1 for calibration requirement)

Estimated Time: 15 minutes for physical replacement, 30-45 minutes for calibration.

Step-by-Step Replacement

1. Locate the Sensor: The sensor is mounted on the windshield, typically behind the plastic cover at the base of the rearview mirror.
2. Remove the Cover: The plastic cover is usually held on by clips. Gently pry it apart or slide it off to expose the sensor and its wiring.
3. Disconnect the Sensor: Unplug the electrical connector from the sensor.
4. Remove the Old Sensor: The sensor is held in a bracket on the windshield. It typically unclips or slides out of the bracket. Be gentle to avoid damaging the bracket, which is permanently attached to the glass.
5. Install the New Sensor: Clip or slide the new sensor into the bracket, ensuring its lens makes full contact with the glass. Reconnect the electrical connector.
6. Reinstall the Cover: Snap the plastic rearview mirror cover back into place.

Frequently Asked Questions (FAQ)