How to clear the collision prevention assist plus inoperative message?

This blog will explain the process to clear the collision prevention assist plus inoperative message in the instrument cluster. Answers to the following questions are also provided here: what are the causes for the collision prevention assist plus to become inoperative? Can you drive when the collision prevention assist system is inoperative?

How to clear the collision prevention assist plus inoperative message?

To clear the collision prevention assist plus inoperative messages in the instrument cluster, connect a diagnostic scanner to the vehicle and perform the control unit adaptation. The message will not be present if the adaptation is successful and there are no fault codes present in the collision prevention assist control unit. Some models may need to be driven to complete the control unit adaptation.

The collision prevention assist plus in Mercedes Benz uses a radar sensor to determine the distance to the object in front of the car. The collision prevention assist plus control unit is designed to apply brakes automatically in order to prevent a collision. This system is different from the Evasive maneuvers assist or Active brake assist.

The radar sensor is located in the front bumper, it is also built into the collision prevention assist control unit. The communication protocol used for the collision prevention assist plus control unit is Flexray. The data transfer speeds of the Flexray are very high compared to the CAN bus protocol and the control unit can react quickly to prevent a collision.

What are the causes for the Collision prevention assist plus system to become inoperative?

The collision prevention assist plus system can become inoperative because of the following reasons:

• Dislocated radar sensor

The radar sensor is located in the front bumper, the position can change depending on the model. The radar sensor uses radio waves to measure the distance between the car in the front so the position of the sensor is very important.

If the sensor is not properly seated in the housing, the radar will not function properly. A fault code is also stored in the collision prevention assist plus control unit. The radar sensor can even malfunction if there is dirt or any object right on the front bumper.

There is no need to remove the radar sensor and clean it, a simple wash will remove any dirt on the front logo or bumper. The radar sensor works differently compared to a proximity sensor. Hence the sensor can be hidden away behind the front logo or in the front bumper.

The front bumper must be removed to check the installation position of the radar sensor, once the radar sensor is properly seated then the stored fault code can be erased. In some cases, a control unit adaptation may be required after correcting the position of the radar.

• Accidental damage

The collision prevention assist plus inoperative message can also appear in the instrument cluster after there is accidental damage to the front bumper. If the damage is not severe, turn off the ignition and press the lock to unlock button on the key to reset the control unit.

The fault message will go away if there is no damage to the radar sensor. However, if the damage is severe then the front bumper along with the radar control unit must be replaced to rectify the concern.

The initial start-up of the control unit is when the radar sensor is replaced. The adaptation of the new radar is also performed so that the system can be fully functional.

• Communication fault

Flexray communication protocol is used to transfer the data between the collision prevention assist plus control unit and the central gateway of the onboard electrical system.

If there is a communication error between the control units, the fault message ‘collision prevention assist plus inoperative message is shown in the instrument cluster. The cause of the communication error can be short or open circuits in the electrical lines of FlexRay.

Flexray system uses a bus plus and a bus minus electrical lines to transfer the data; there is a universal, central, and end node in a branch of FlexRay. The universal node control unit will turn off the branch if the communication is missing.

It is important to check the resistance in the FlexRay branch before replacing any component. The measured resistance reading at the control unit can identify the break in the branch that’s causing the communication fault.

Once the communication with the control unit is established, the collision prevention assist plus an inoperative message will not be shown in the instrument cluster.

Can you drive when the collision prevention assist system is inoperative?

Yes, it is possible to drive with collision prevention assist plus message inoperative in the instrument cluster. The collision prevention assist plus is a driver assistance system and does not impact the ability of the vehicle to be driven.

The engine can still function despite any communication issues present in the vehicle that is leading to the fault in the collision prevention assist plus control unit. The functioning of the active brake assist is also dependent on the signal from the radar sensor. Therefore, if there is an object in the path of the vehicle, automatic brakes will not be applied as both systems are inoperative.

Drivers may also need additional brake force because the pressure in the system is not increased to avoid a collision with the object in front of the vehicle. It is also possible to turn off the collision prevention assist plus system from the menu in the command operating system.

After the driver selects the off option for collision prevention, the system will stay off for the entire trip. The collision prevention assist plus will turn on again after the vehicle is started for the next trip.

What is the difference between the collision prevention assist and collision prevention assist plus system?

The collision prevention assist system is found in the old Mercedes Benz model. It used a similar radar sensor but the range was only up to 80 M in front of the car. If a stationary object is detected within the radar range, an audible warning tone alerts the driver.

If the driver continues to drive, the collision prevention assist system will work with the Pre-safe to apply brakes and prevent a collision. 

The collision prevention assist plus is the latest version of the collision prevention system, it not only monitors the distance to the object in front of the car but also monitors its speed. So if the vehicle suddenly brakes in the front after a green signal or on highways, collision prevention assist plus can take action to prevent the collision.

Conclusion

This blog explains the method to clear the Collision prevention assist plus warning message in the instrument cluster. The cause that can lead to the malfunction of the collision prevention assist plus system is also explained. The warning message can also clear on its own if there is no damage to the radar sensor.