I have a 2012 C300 4matic sedan with KG. The car is new - bought at the end of the model year in late August 2012 - total mileage 1956 - 400+ of which have been put on by the dealer.

I began developing intermittent KG problems in November, slowly increasing in frequency to the point where is was annoying enough to bring the car in for service in December.


Since Christmas the car has been back to the dealer 4 times and for a total of 21 days. They have replaced the key batteries, the keys themselves, the KG computer module. The last time the dealer had the car was for 8 days - it was returned to me today with the dealer saying they tried everything they could think of but they could not reproduce the problem. Within 1/2 hour of the car's return I was locked out of it yet again!

So 2 scenarios -

1. if I leave the car unlocked and get in and press the start button it will work sometimes and sometimes it will say KEY NOT DETECTED. Even if I am waving the key in front of the speedometer. Once I press any button on the smart key itself, the car will now recognise the key and I can start the car.

2. If the car is locked, sometimes it will recognize the key and allow me to unlock the car and sometimes I will walk up with my key in my pocket and I am unable to open any door or the trunk and the speedometer will say KEY NOT DETECTED. Once I press any button on the smart key itself, the car will now recognise the key and I can enter and start the car.

Problem happens with both keys, in multiple locations (garage, driveway, office parking lot, local store) - doesn't seem to car what I have in my pocket or on my person - iPhone5, home security system key RFID key fob (works or fails with it or without it). I don't have a metal plate or screws anywhere in my body (that I know of).

Today when the car was dropped off, I tested the door action and all was fine. Then I drove it around to the back of my office building and locked it. I went upstairs and came back down 1/2 hour later to go out and pick up some lunch. LOCKED OUT. All 4 doors. Took key off key ring with security FOB and gave it to my friend who walked 50 feet away. Still locked out. Pressed unlock button on key - doors unlock. Then I drove 1 mile to a store, parked and locked the car and went inside. Came out 10 minutes later and same thing - LOCKED OUT. Gave my friend my cell phone and he took it 50 feet away - no joy. Pressed the button and drove back to work. One hour later back downstairs - everything exactly the same - same key, same jacket, same pocket and walk up to car and Voila! KG works.

I have video'ed this happening 3 times and sent to the dealer. It's probably happened a total of 30+ times. Dealer was able to have car fail initially but now won't fail for him.

Any ideas?

I have no idea, it is probably the key sensors in the car (which should have been replaced or fixed by the dealer after the 2nd or 3rd visit) or the connections to the sensors. Does the failure appear to coincide with environmental factors (only when it is cold outside or windy, or whatever)?

Regardless, with the video, you have grounds to return it as a lemon (in MA it is just 3 returns for the same issue). You will probably be able to get 2013 with the 3.5 DI to boot. They will probably push back and say something like we'll refund your money for the keyless option, but I'll bet if you push you can make the return happen. This really shouldn't happen on a car like this.

They are now waiting on response from factory tech rep as to when he can visit the dealer and then they'll want me to give them my car again.

Failures didn't start happening until weather turned colder but there is no direct correlation. AFAIK they have not replaced any sensors - not sure why - if it were me I would be throwing parts at this. They have done extensive diagnostics on all the computers and put their shop foreman on the problem but like I said - in 8 days they could not recreate it and in 1/2 hour it happened for me.

I had the same type of problem right after I bought my CPO 2010 back in November. The dealer fixed it right away.

Just in case you didn't know, you won't be stranded, you can pull the start button off and use the key like it didn't have the option. Unfortunately, I forgot this when it happened to me. i had to sit and talk nicely to my car for about 15 minutes in the parking lot of a restaurant before it let me start it up. I got a couple funny looks from passers by, but nothing serious :-)

Just out of curiosity, did you try the other doors when this was happening?

All 4 doors

Well, mine was fixed with a software update so I doubt yours is the same issue but you never know.

This is my sixth MB and depending upon how this plays out it could be my last. It is the first time I have ever had a problem with any of my cars - all bought new - and I'm a bit stunned to find the dealer incapable of fixing the issue.

I understand that the intermittent nature of the problem (sometimes it works fine for days then fails) makes it hard to diagnose - yet for the problem to reappear within 1 hour of dealer returning the car to me after saying they couldn't replicate the problem in 8 days belies belief.

I would write a letter to the dealer stating what you are saying above and simply ask to have your car replaced and let them figure it out on their own time while not inconveniencing you. If they are not willing to give you a new car, you may have a lemon law claim just the same.
Good luck.

I think I fixed it on my own - after a month with the gang that couldn't shoot straight ( dealer service dept ).

It's a bad key.

I've been able to reproduce it a dozen times now in one morning - go to car with the bad key and either be locked out or have car refuse to start. Then go in the house and swp keys and voila! Doors open, car starts.

They have had both keys all 4 times they have had the car - not sure why they couldn't get to the same place I did. Also - they replaced both keys - so either this is a problem with 1 new key (possible) or more likely a problem with some central security computer (EIN?) that slowly "forgets" about a key.

It's time to apply the lemon law

Not to insult you, but you do know where the "open" and "close" sensors are located, so as not to touch on both at the same?

I have a 2012 C350 4matic, picked up new at the end of August 2012. I just had my first experience yesterday of the car not finding the key. It happened during a trip 15km ride. Was able to get into the car, start driving and then the nuisance message started appearing. The key was in my pocket.

Will report back if it is a persistent problem and any experiences with the dealer.

I've had similar (NON)issue on my 2 month old car. This is normal!!!
Have you ever had an internet drop out for a very brief moment?
Have you ever had cellular signal drop our for a moment?

Yes, I know how to open the door and how to lock it.

Problem solved I think.

It's a bad key. One key works, the other doesn't - the results are completely predictable and repeatable. I've done it about 15 times now in 2 days. Why the dealer could not figure this out over a month and 4 extended visits totaling 21 days is beyond me. I'm not thrilled with their level of competency - they must have been testing the keys together on the same plastic tie wrap (that's how they were returned to me) - so the car always found the good key and not the bad - that's why they couldn't reproduce the problem this last time.

It's possible the problem is not the key itself - maybe whatever the key registers itself too (EIS?) - because these are supposedly new keys specially made and programmed in Texas (that's what they told me) and the old key failed as well..

Will talk to dealer again tomorrow.

Just as a heads up -

Mercedes acknowledges that there is a software issue with KG - not sure if it's C300 only.

They are working on a fix.

Well, I have a new E350 and this problem has been driving me crazy! I've been trying to troubleshoot it myself and have not taken it to the dealer yet. I decide to check the forums first and saw your issues which are very similar to the intermittent problems I have experienced.

Can you provide any more detail on the fix? Any timing? It sounds like going to the dealer is not going to help until the fix is out. Do you know if Mercedes has relayed the message to its dealer network?

I had a similar issue with my CPO 2010 E350 right after it was bought about 3 months ago. It had to go back in for registration and and install and I mentioned it to them and they updated a SW fix and I haven't had a problem since. Maybe the Es and Cs are having different issues.

KG has been available since 2007 on other MB models. This is not new technology.

It is indeed time for a lemon law activation.
Start your paperwork.

We have had this issue very sporadically with our 2012 C350. It's probably happened about 4 times in the 6 months we have owned the car. Doesn't cause any problems as pressing a button on the key makes it work again. I will ask it to be looked at the first service. Our car has two drivers switching quite often and I often wonder if its because the car "learns" the other driver's key and gets stuck searching for that key and not recognizing the other (just speculating, software can do stranger things).

There is another thread on a similar issue in which, I believe, replacing the EIS seemed to solve it (I have no idea what the EIS is). Although I think that was more of an issue of the car not starting not just gaining entry.

My dealer called me today to tell me that the factory tech rep had been there yesterday and that the issue is acknowledged by Mercedes and referred to the factory in Germany for a fix. He says they expect to deliver a software upgrade with fix by end of next month - ie; end of March.

Mercedes is refunding me the cost of the KG option - I have accepted this with the caveat that it does not prejudice any recourse I might have to lemon law in the event there is no resolution by then. At this point I have one key that seems to work reliably for KG (for now) and one key that will not work at all for KG. Since I have functionality as long as the one key works, I am willing to be patient a while longer. We'll see what happens.

I suggest you try both keys ONE AT A TIME, with each key separated by at least 50 feet. My dealer had my car for 8 days and could not reproduce my problem because they had both keys tied together the whole time! The car always "saw" the good key and never failed. Once car was returned to me and I separated the keys, the KG failed within 1/2 hr - as soon as I tried to use the bad key. Worth a try...

You may have a single bad key that gets used sometimes in this round-robin usage scenario.. read my post(s) above.

Also - the issue is the same for door opening and for car starting - if the doors are locked and KG fails you will not be able to open them until you press the button on the key and then the car will "recognize" the key and open the door and the starter button will work too. If the car is unlocked and the key is not recognized then the car will not start when you press the starter button in the dash until you press the button on the key etc..

It's the same problem - "Key Not Detected".

Hello. I have a CPO 2012 C250. I am very pleased with the auto. However the KG malfunctions often just as "Emmotto" described in January 2013. We are all currently scratching our heads. The dealer has replaced the batteries several times,and installed a software patch. If anyone has found a solution to intermittent KG problems described by Mr. " Emmotto" please advise. I'm currently driving my third dealer loaner car. Thank you.

No solution that I know of yet - mbusa maintains that it is being worked on. In the meantime I have accepted a financial settlement instead of going thru lemon law - less hassle. This car is a short term lease (2 yrs) so I will be rid of it next year - if I had purchased this car I would have definitely pursued lemon law.

The real question is do I buy/lease another Benz in the future - going to be hard to purchase the KG option on my next car after this experience....

Thanks for taking the time to respond. You're a fine man ! I've also had the KG controller replaced. However, the KG failed again the same day the dealer installed this repair. Arghhhh . I hope to post the solution ,eventually , to help others. Other than KG issues , the car has been fabulous . Thanks again.