Hi

I have a 2021 GLS 450. The car was transported on a car hauler and now has a collision detection notice that appears on the screen. I have tried to get rid of it without success. There isn't anything I can find on the App or in the cars system under Vehicle or anything else I could find. I have read that disconnecting the battery might get rid of it, but don't want to do that after my wife's GLC's side mirrors stopped working when I did that to her car. Others say take it to the dealer. I can, but prefer not to. Does anyone know if an OBD scanner from say isoft will do it? I could not see that function listed on their site. Any other ideas, besides take it to a dealer?

 

### Understanding the Collision Prevention Assist System in the 2021 Mercedes-Benz GLS 450

The 2021 Mercedes-Benz GLS 450 is equipped with the Collision Prevention Assist Plus (CPA+) system as part of its standard Driver Assistance Package. This feature uses front-facing radar sensors and cameras to detect potential collisions, providing warnings, partial braking assistance, and in some cases, automatic emergency braking. If the system detects a fault—often displayed as "Collision Prevention Assist Plus Inoperative"—it disables these functions for safety, which can also lead to reduced throttle response or power limitations. This is a common issue across Mercedes models from 2015 onward, including the GLS-Class, and is frequently triggered by low battery voltage, dirty sensors, or software glitches rather than a major mechanical failure.

### Common Causes
Based on reports from Mercedes owners and technicians:
- **Low battery voltage**: The system may shut down if the battery has been disconnected, drained, or is weak (common after long periods of inactivity).
- **Dirty or obstructed sensors/cameras**: The front radar sensor (behind the bumper) or windshield camera can get covered in dirt, snow, or debris.
- **Sensor or wiring faults**: Intermittent issues with the radar unit or connections.
- **Software calibration needs**: After repairs or battery work, the system may require recalibration.

### Step-by-Step Guide to Reset the Collision Prevention Assist
Resetting the system often resolves temporary faults without tools. If these don't work, professional diagnostics are recommended. **Important safety note**: Do not drive if the warning persists, as it disables critical safety features. Always test in a safe area.

1. **Check and Clean Sensors/Cameras**:
- Park in a safe, well-lit area.
- Locate the front radar sensor: It's embedded in the center of the lower front bumper (behind the grille).
- Gently clean it with a soft cloth and mild soap/water—avoid high-pressure washers, which can damage seals.
- Clean the front camera: It's mounted behind the windshield, just below the rearview mirror emblem (under the Mercedes star).
- Wipe away any residue, bugs, or film. Dry thoroughly.
- Start the vehicle and check if the warning clears after a few minutes of idling.

2. **Battery Voltage Check and Recharge**:
- Use a multimeter to test the battery voltage (should be 12.6V+ when off; 13.5–14.5V when running).
- If low, charge the battery fully (use a trickle charger if needed) or replace it if it's over 4–5 years old.
- Disconnect the negative battery terminal for 10–15 minutes, then reconnect—this can reset minor electronic glitches. (Note: This may require re-entering radio codes or settings.)

3. **Drive Cycle Reset (No Tools Needed)**:
- Start the engine and let it idle for 5–10 minutes with all accessories off (AC, lights, etc.).
- Drive the vehicle at highway speeds (above 20 mph) for at least 20–30 minutes in varied conditions (e.g., straight roads, light traffic). The system self-calibrates during this time.
- Avoid stop-and-go traffic initially to allow full sensor initialization.
- Press the "OK" button on the steering wheel to acknowledge any persistent warnings during the drive.

4. **Manual Activation Toggle**:
- In the instrument cluster or central display, navigate to **Settings > Assistance > Collision Prevention Assist**.
- Toggle it off, wait 10 seconds, then turn it back on.
- If the option is grayed out, proceed to the drive cycle or diagnostics.

5. **OBD-II Scanner Reset (DIY Option)**:
- Use a compatible OBD-II scanner (e.g., iCarsoft MB V3.0 or YOUCANIC for Mercedes-specific codes).
- Plug into the OBD port (under the dashboard, driver's side).
- Scan for codes in the "PRE-SAFE" or "Radar Systems" modules.
- Clear codes and perform a "variant coding" or "calibration" if prompted (this recalibrates sensors).
- Cost: Basic scanners start at $50; Mercedes-specific ones are $100+.

### When to Seek Professional Help
If the warning returns after these steps:
- Visit a Mercedes-Benz dealer or certified independent shop for Xentry diagnostics (Mercedes' official tool). They can recalibrate the radar (costs $100–$300) or check for deeper issues like a faulty sensor ($500–$1,500 to replace).
- Common in cold weather or after front-end work—don't ignore it, as it ties into PRE-SAFE systems.

### Prevention Tips
- Regularly clean sensors during washes.
- Maintain battery health with annual checks.
- Update vehicle software during service visits (over-the-air if available).

This should get your GLS 450's system back online safely. If you provide more details (e.g., exact warning message or recent events like a battery swap), I can refine these steps. Drive safe!

 

Grear post. Glad it's not somethhing I have to deal with.
Tech is cool when it works, umtil it doesn't.

 

This class of issues can be life threatening. Ghost brakings hazards may result.

Refer to dealership services for factory grade troubleshooting and when that fails... request to have the bumper harness connection inspected or replaced & waterproofed.

Voltage transient are very effective at disrupting these systems quietly ithout fault logged. Meaning do not expect helpful DTC's for effective repairs. The root cause is kept concealed.

 

Try a hard reset first. Turn the car off, get out, lock it, and leave it alone for about 30 minutes. This lets all the computers fully go to sleep and can often clear glitches like that.
If that doesn't work, a basic OBD scanner won't have the function you need. You'd need a more advanced one like an iCarsoft or Autel, but even then it's not guaranteed. The dealer is the sure fix.