I’ve had a Mercedes Benz B-class (W246) now for 3 and a half years and on the first two occasions I had the B-class serviced within a few days I have had “Pre-safe functions limited” message happen shortly afterwards. Mine did the pre-safe functions limited message several times, each time after it had been serviced even though I had it serviced by Mercedes.
This happens at random intervals, speeds or with the car on a hill or on a flat and puts the car in limp mode. This can be quite dangerous if you are overtaking when this happens as its like someone has switched the engine off.
As I said before this has happened on two sets of occasions both after the car had been serviced by Mercedes themselves and led me back to Mercedes to try get this fixed.
The cause of this problem is to do with the connection between the engine ECU and wiring loom. The pins which make this connection inside the ECU socket are not a tight enough fit and on my car the pins had to be removed and larger items installed. This ties in with the servicing, as at service the ECU is unplugged while the service is performed and the car is connected to the workshop computer. Ironically this is to see if there are any problems or errors in the Ecu.
If your car is coming up with the “Pre-safe Functions Limited” error then first thing to check is the wiring loom & ECU connection and look at the pins inside. If there is any marks or discolouration, such as arcing marks, on the pins then you need to take the car to Mercedes for repair/replacement.
Ok initially Mercedes told me this was the Throttle control switch which controls how much throttle the ecu see’s on the fly by wire accelerator. Mine was changed twice and both times it failed to solve the issue and I am sure the Mercedes Benz garages changed it to get some money from Mercedes warranty.
Mercedes are aware of this issue as I have seen official Mercedes workshop documentation on this issue, however I was unable to get the document number or a copy of this “internal” document as the service manager was very coy whilst showing me this.
Once the pins are changed for larger items this solves the issue and since it was done I have not had a single recurrence of the pre-safe functions limited problem.
If your B-class does this then don’t ignore the issue, even though a restart of the car clears the fault. This can be dangerous and if your car does it once it will do it again.
Also it will most likely be reported in the ECU as a throttle position sensor problem. Initially Mercedes Benz replaced mine, not once but twice but the problem still remained after the this was changed.
If your car does this then my advice is to get Mobilife to come and pick it up (if you have it) or take it to a Mercedes Benz dealership.