Mercedes Benz B-Class (W246) “PRE-SAFE Functions Limited”

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.

Mercedes Benz PRE-SAFE Functions LimitedIf  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.


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11 thoughts on “Mercedes Benz B-Class (W246) “PRE-SAFE Functions Limited”

  1. Thank you very much for the thoroughly information concerning this PreSafe warning fault. This just happened to my mothers W246 (2012) and I have now adviced her (over the phone) to have the car serviced at the MB Dealer pronto 🙂


  2. Great info thanks. This has happened to me 4 times now. The first time it rectified it’s self when switched off, it does every time! Next time I took the car to MB to check out. They did several updates and said it was ok, not so it happened again. So back to MB for a diagnostics report. They said it was the throttle position sensor and would require a new electrical loom replacement £268, I didn’t have that done. Again the car was ok for a hundred miles, Then it happened again. This is a very dangerous thing to have happen and has the potential to cause a major accident. Last time it happened to me was on a roundabout with fast moving traffic, I had to turn left in limp mode (not the direction I wanted) and got a few beeps for my trouble!! I’m taking my car to my local garage armed with your info because I trust them more than MB. If this is such a problem with the W246 models (which I believe it is) why oh why don’t MB admit to it and rectify before it proves fatal to someone!!!!! It’s a fantastic car in every other respect and I don’t want to get rid of it unless I can rectify this problem. If not it will be back to Volvos for me, besides they are more comfy! Thanks to More people need to report this problem it’s important for all our safety. MB own up you have a problem!

    1. You’re welcome. It is indeed dangerous and one time mine did it it caused pandemonium behind me. Funny you should mention the throttle position sensor as I had my car recovered by Mercedes and this was the first thing they changed as they said the signal from the throttle position sensor was either zero or one hundred percent. No inbetween! This is of course because of invalid readings because of the short circuit. Mercedes might not recognise the fault I have described but I have seen the paperwork from Mercedes regarding this but the technician who showed them to me would not let me have a copy or photograph them.

  3. Cheers, I’ll let you know the outcome from my local garage. But if MB know this is a problem why are they not doing anything about it, except getting more money for more checks they do not resolve! Obvious….more money. But on a very serious point this is a dangerous fault in an otherwise great car. The fact that it seems hidden in their diagnostic reports is a worry!! They know the problem is there so why not fix it. I spent a lot of money on this car for my retirement thinking it would last a great deal of time, but safety is one thing I won’t compromise on. It pisses my off no end to know they can be so flippant with my cash investment. But hey do they care? well for sure if enough people complain and kick up a stink about this, which I doubt. Just like many other manufactures of cars with faults people will except it without question until it effects them in an adverse situation. Thing is it could be fatal. 5 star euro testing is fantastic in every respect and we all probably look to this before we purchase a car. But if it doesn’t show this problem is there it may as well be 2 star no matter what the other safety levels are. I truly hope people read this and get on to BM about the problem and make them do something about it. It’s a simple resolution to a simple problem as you put it which I’m sure is true. Get that paper work and shame them if you can. Thanks again.

    1. Is yours an early 2013 model? I am led to believe it only affected certain batches of the car. Mine had the monochrome dash where as the later models had coloured dash. Be interesting to know if the later models are afflicted with the same issues. Mine was fixed under warranty so no money lost.

      1. Mine is a 2012. I’ve had several in the past, first being a 2008 which never had this problem. It did however have a glow plug problem which MB tried to fix and snapped all 4 heads off, so engine had to come out! A had a written quote for £700 but when it was fixed £1300 was asked for!!! They settled at £800. I don’t know why I keep going back.

  4. Hi mykp, sorry to hear it’s happened to a family member let alone anyone else. I took mine to my local garage for an independent diagnostic report and it came back with the same result but more telling they told me this is a known problem to Mercedes and therefore they wouldn’t touch it. I’ve been in touch with the Mercedes dealers that sold me the car and explained the situation. Two weeks on and one reminder still no response!!! They are looking into it so I’m told!! The only thing I’ve been offered so far is a fix costing £265 but parts are just £ 13.00.! How do they work that out?? I’ve looked on MB forums and everyone seems to except it’s rain, snow, radar interference or dirty sensors. What is wrong with you guys, this is a Mercedes problem that for them is an easy fix at very little cost. Complain, complain, complain. You paid enough for the car in the first place, it should not happen….plain and simple. This car is one of the best on the market for me at the moment in a great many respects but I’m looking at a Volvo V60 estate tomorrow, enough is enough. Why….. because Mercedes can afford to ignore little people. I wonder what they would say to Boris Becker!!!!

  5. Hi 2 years on and your post is still of great value i suffered for a whole year the identical problem you drscribed even had the throttle changed this seemed to make the car even worse .I read your post showed this to my mechanic who seemed a bit skeptical but agreed to disconect the ECU and look at the plugs he never found anything out of place.and simply reconnected the plugs .that was 2 months ago .i have had trouble free driving ever since .So thank you so much for Sharing..

  6. Same problem. Great post that succinctly advises. Is it possible to disconnect and reconnect the loom to the ECU yourself?

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