MB Medic Mercedes-Benz Repair Guides, Service, How To DIY
Last updated:
In this guide (6 sections)
Mercedes Blind Spot Assist is a safety feature that warns the driver if another car is in the blind spot. This feature has been optional in many Mercedes-Benz models since early 2010. This guide will look at some common problems with Mercedes blind spot assist and why you may get a warning message such as “Blind Spot Assist Inoperative” or “Blind Spot Assist Currently Unavailable. See Operator’s Manual”
Blindspot assist Inoperative Message
It is not uncommon to get a blind-spot assist deactivated warning message on the dashboard. Typically, this happens during heavy rain. You may also get several other warnings on your dashboard, including:
- Pre-Safe Functions Limited
- Active Lane Keep Assist is Unavailable
- BAS and ESP lights on
The most common causes that trigger  Blind Spot Assist Unavailable  are:
- Heavy rain
- Inclement weather, including snow and fog
- Extremely high temperatures
- The radar sensors in the rear bumper are dirty
- Driving too close to lane crash barriers or dividers
- Driving next to a truck for an extended period
- Interference from other equipment or nearby transmissions (TV and radio stations)
In addition to the message on the instrument cluster, you may also notice the yellow triangle on the side mirror staying on. If you are expecting Blind Spot Assist problems, find a safe location where you can stop and restart the vehicle.
Troubleshooting Mercedes Blind Spot Assist
Avoid driving through water puddles in severely inclement weather. Wait for the rain to stop, restart the car, and check if that resets the warning light. The system should reset independently; generally, there is no need to worry unless the blind spot system remains disabled the next day or this problem happens frequently.
If you get an error message saying that blind spot assist is inoperative every time it rains, you have a problem that you need to address. The blind spot system is not supposed to turn off every time it rains, but infrequent malfunctions are common.
If the blind spot assist remains disabled, check fuses. If the fuse has not blown, check the following:
Water in Radar Sensor
The most common problem is that the sensor can get damaged by water. If so, the blind spot assist sensors behind the rear bumper must be replaced. When the blind spot radar sensors in the rear bumper fail, the turn signals may also stop working. If you are getting a “blind spot inoperative” message on your dashboard every time it rains, remove the radar sensors on the rear bumper and ensure they are properly sealed. Water may be entering the radar sensors, triggering the message on the dashboard.
Sometimes, when the blind spot assist stops working, the wiper blades may also stop functioning. If you are experiencing these two problems, check the fuse for the wiper blades. Most likely, the fuse has blown. Once you replace the fuse, you should also gain access to the radar sensors in the rear bumper and inspect them. Make sure the radar sensors are completely sealed. Water entering these sensors can blow the wiper fuse because the same circuit protects them.
Dirt on Radar Sensor
Dirt on the radar sensors can prevent the blind spot assist from working properly. Remove the sensor and clean it if necessary.
Steering Angle Sensor
Another issue that makes the blind spot assist inoperative is the steering angle sensor mounted in the steering column. This sensor determines the position of the steering wheel. If it fails, the blind spot assist cannot get data and determine the steering wheel’s position.
Towing
If you tow a trailer or another vehicle, you may get “Blind Spot Assist Currently Unavailable. See Operator’s Manual.” Once you remove the trailer, the system should be back to normal operation. While you have a trailer attached, do not rely on the blind spot assist system; always check the blind spots before you change lanes.
Buy Mercedes Blind Spot Sensor
Replacement Mercedes-Benz blind spot radar sensors can be purchased from Mercedes-Benz dealers or online retailers.
You can check current prices for Mercedes blind spot radar sensors on eBay.
How Does Mercedes Blind Spot System Work?
First, we need to understand how the system works. Radar sensors installed on the rear bumper detect vehicles in the blind spot areas. The radar sensors are behind the bumper. They are not the same as the parking assist (ultrasonic) round sensor you see. When a car is detected, Blind Spot Assist will warn the driver via the triangle in the side mirror. If the Blind Spot system detects that you are getting too close to the other car, it will apply the brakes on one side of the car to move you away from danger. Blind spot assist will only apply brakes if you are about to get into an accident, such as when ignoring the warning or trying to change lanes when another car is next to you. If you are a careful driver, hopefully, you will never experience blind spot assist in action.
You should always check the blind spot when changing lanes.  It is important to understand that the Mercedes Blind Spot system works at speeds over 20 mph. Blind Spot Assist does not work if you travel less than 20 mph.
Also, remember that motorcycles may not be detected, or if they are, they may be discovered too late. Radar sensors are installed on the front bumper and front grill, but those sensors are not used for the Blind Spot system, but for Distronic only.
It also does not detect vehicles that are traveling significantly faster than you. Blind spot assist may not identify other cars if the lanes are very wide. That’s why the yellow warning light on your side mirror may stay on when driving slowly. The yellow triangle on the side mirror should turn off once your speed is over 20 mph. Keep in mind that blind spot assist does not work when backing up. Blind spot assist monitors 10 feet behind the vehicle to the sides.
How to turn off /on the blind spot assist
If you will tow a trailer or another vehicle, turn off the blind spot assist. To do so, you can use the buttons on the steering wheel until you see the option for Blind Spot Assist. Enter this menu and turn it off using the OK button on the left side of the steering wheel.
What Scanner Can I Use to Diagnose Blind Spot Assist Problems on Mercedes-Benz?
By this point you have waited out the rain, checked the fuse and cleaned the rear bumper, and the message is still on the dash. What the dash will not tell you is which of the two radar sensors is unhappy, because a water damaged sensor, a dirty one and a sensor knocked out of alignment all put the same warning in front of you. A scanner that reads the driver assistance modules rather than the engine alone, such as the YOUCANIC UCAN-II Pro, pulls the fault from the sensor itself and tells you which side is reporting and whether the system is asking for a replacement or a recalibration.
That is worth knowing before you spend anything. The sensors sit behind the rear bumper cover, one for each side, so reading the fault first is the difference between ordering the right sensor once and ordering the wrong one twice.
Before you buy parts, scan every control module, not just the engine. It shows the root fault behind the warning lights and tells you whether the problem is still active. See how to do a full health check.
