Synchronising the single button IR Mercedes Key

synchronising the mercedes Infra Red key

This is quite a simple task if you have changed the batteries or for whatever reason the key no longer correctly locks and unlocks the car as it used to.

There are principally three variations on a theme and these are outlined below:

The following instructions are for the square flip out key (not the later DAS pear shaped keys)

Model W124 (with code 885)
Model W124 model year 01-03-96
Model W129,W140,W202 model year 01-12-94 up to 31-05 97
Model W210 model year up to 28-02-97

Point the remote key towards the rear view mirror receiver.

Press the remote button just once.

NOTE: For vehicles model year up to 12-93 – lock or unlock car with the remote key flip out blade within 30 seconds in door lock

Mercedes C class models from Jan 1994 with control modules 202 820 40, 26 and 202 820 43 26. Turn on ignition to position 2 with folding key within next 30 seconds.

W210 E class cars as from December 1994 with part numbers:

210 820 21 26
210 820 22 26
210 820 27 26
210 820 28 26
210 820 32 26

Mercedes remote locking receiver

Point the remote key at the interior mirror, press remote button twice then turn key in ignition lock to position 2 (within 30 seconds).

Test function of key, it should now be synchronised!

If the key fob now locks and unlocks the doors correctly, but the red and green lights flash alternately red/green unusually on the rear view mirror – replace the batteries in the key fob remote.  This alternate flashing is the Mercedes indication of low batteries in the remote fob/flip key – it is not widely publicised.

You can watch a video of how to change the key fob batteries here.

44 thoughts on “Synchronising the single button IR Mercedes Key

  1. car fault for two weeks installed new batteries in remote and problem solved thankyou for the info on flashing lights mirror

    1. Hi Brian,
      Pleased it helped you out. There is not a great deal written about this tell-tale warning that you need to change your batteries!
      All the best

  2. hi ! sorry to confuse u …
    i hve one key but i bought one from ebay exact no. 129 760 00 01/06 siemen

    how to make it work with car 92 mb w140
    300se ?

    i hve to buy a tool that connect to computer de-soldering the chips ? or some or better tool or method ? hong kong cost hkd3000 almost usd400 !

    1. Hi Leung,
      The key as you say is coded to the ECU. The factory way would be to program a blank burn-once eprom raid key to the VIN. There are obviously ways to copy the existing chip (once desoldered) and program a new blank IC and solder it to the new key PCB as a direct copy. I would imagine the expertise to do this and sourcing the equipment to do it would exceed the dealer cost of a replacement key.
      Sorry I cannot be more help.
      All the best

  3. Hi there. Are there some more unknown moves to syncing that I am not aware of or am I just doing something wrong?! I have 1995 MB w202 c180, car has been sitting now for the 5th month due to not being able to start it, IR green/red lights just flashing in the rear view mirror. Have tried the syncing with multiple car batteries and many sets of new batteries for the key fob and absolutely nothing. Am I missing something somewhere?! Was using the car daily for the last 5 yrs and now after the battery change nothing. Any tips are truly welcomed.

    1. Hi there,
      Just try the following reset proceedure and report back.

      To reset immobilizer. Unlock the car from the trunk then lock it. Then unlock drivers door. This should reset the security system.

      All the best Steve

      1. Many thanks for answering so quickly. Damn, after posting I realized that I should have explained the current state of the car with more details. My trunk lock had been on it’s last legs for years. Meaning that it was lockable/openable using key fob/central locking. The key though did not turn and was never able to lock/unlock the trunk like that. As time went by, the lock was dying on me and I used a dart and quite strong slap to make the locking mechanism move and open the trunk. This year though I had to remove the lock completely and trunk has a see-through-hole til this day. Just could not open it and had to take measures. Secondly I have read about driver’s doors, but mine has another IR receiver in it and the door is not lockable with a key, passenger’s side is though. What I have also noted, is that the car does have central locking which has been working fluently the whole time I have owned it. Yet if I lock or unlock the passenger’s door from outside with the key, other doors remain the same. As the car sits permanently unlocked, sitting in it and locking the driver’s door from inside, same thing – other doors do nothing. Yet it does have central locking button in the middle console and from there, all the doors do lock and unlock nicely. Your suggestion to close the trunk makes somewhat sense, but not possible to try in this case. If I’ll go and buy another replacement lock, install it…is there really hope to be able to reset the security system and finally get somewhere or do I have any other options to try?!?

      2. Hi there,
        As that is the default reset proceedure I dont really know what else to suggest. Maybe you could locate the electrical switch wires in the boot and drivers door, that would normally send positional signals from the mechanisms to simulate locking and unlocking the locks in the order required, without the physical locks in place. You really need to at least try to perform the correct reset proceedure before looking for other issues that may or may not exist. I would try substituting the use of the drivers door in the sequence with the passenger door that in your case has the emergency key slot.
        Good luck, do let me know how you get on.
        All the best

  4. Hello,

    I am in the same shoes than prt, except that I have a working trunk lock. First, I have tried the casual syncing method, result was flashing lights in the mirror, nothing else. I bought new batteries for both key and changed it. Now it does not flash, but same thing, nothing happens.

    After I read your trunk trick. Car was fully locked, went there, opened the boot, then locked it. After driver’s side door, press the key 2x pointed at the mirror, ignition 2nd position. Then nothing happens when I press the button, not in any angle. I really don’t know what else I could try.

    As an additional info, my key’s IR light does not flashing through the camera. So I guess end of story with that, I need to change the circuit board. :/

    1. Hi Laszlo,
      Often the problem lies in the little push switch that is on the PCB inside the key case, over time this gets some real abuse and its only a fragile thing. If you are handy with electronics you can replace this with a fine soldering iron. You will find the kind of thing here.

      Hope that helps,
      All the best

      1. Thanks for the reply Steve.

        I checked that button has 2 legs, mine on the PCB has 4, is this the correct one for sure?


      2. We changed it both and fixed the battery connector and surprise. Working! Thank you very much Steve!


  5. My w202 c180 1995 remote doesn’t unlock the doors after it was locked by the same remote. I unlocked it manually from the passenger side now the engine also doesn’t start. Is the engine start failure because of remote malfunction or something else and how can fix?

    Update : Everything ok at this time. Just tried the above synchronisation method and it worked. Anyway thank you so much for the guidance.

  6. Hi my key part number is 1407601406 -changed batteries and now the key fob will not work. I get an IR light if button held for over a second or two. Its a 1996 Sl320 R129 Mercedes Benz. Which procedure do I need to follow?

  7. 97 Mercedes c280. Single button key fob starting numbers 201 Replaced the battery tired several times via the manual to reset found out when I tried to turn on the interior light wouldn’t go on checked the fuse blown. Replaced now sensors in rear view worked no problem. Now works to lock and unlock but now when I goto start it tries to turn over almost run then shuts off. Please help man with any possible ideas

    1. Hi Ricky,
      Are red green alternate lights flashing in rear view mirror when you crank engine? If so alarm is not deactivated. The alarm controller could have glitched – You can reset the system by unlocking the car from the trunk/boot then lock it. Then unlock drivers door. This should reset the security system, if still no joy try synchronising the keys again after the reset.
      All the best

  8. Hi guys! May I ask you for your advice: I have a ’07 mercedes W219 and one of my key has stopped working since few days – the LED light & I.R. is still working but the key does not open the care, it fits into the ignition but won’t turn. I have replaced the batteries but it still does not work. It seems that the key lost the program, is there any procedure to program/resync the key again? Thanks in advance.

    1. Hi Andre,
      The resync would not have any effect it turning in the ignition slot or the release of the steering lock etc, this is probably a faulty key and a new one required.
      All the best

  9. Hi,

    I have a Mercedes w202 1997. I bought it few weeks ago – everything was fine until yesterday. I managed to lock/unlock the car in the morning, but couldn’t unlock it using the fob in the evening. Was able to drive back home using emergency key (to unlock the car), however, the alarm was going off all the way. I have changed the batteries and now need to synchronise the fob.. so far I have tried following procedure listed in the manual – simultaneously pressing ‘lock’ & ‘unlock’ buttons for 6 seconds; unlocking the car using emergency key, pointing the fob to the rear-view window and pressing ‘unlock’ button twice (and once), putting the fob into the ignition and turning to the 2nd position. However this doesn’t seem to work…any ideas?…Many thanks in advance! 🙂
    I forgot to mention that the LED & IR are working – tested with a camera.

    1. Hi Eve,
      The immobiliser alarm does not seem to to working correctly as in an alarm condition the vehicle should be immobilised from starting by a relay down by the accelerator pedal. So it should have not started with the alarm running – is the door locking working from the key (all doors lock unlock – signifying the door pump circuit is working correctly)
      It may be worth just trying the reset procedure just in case its got itself confused! Just try the following reset proceedure and report back.

      To reset immobilizer. Unlock the car from the trunk then lock it. Then unlock drivers door. This should reset the security system.

      All the best

      1. Hi Steve,

        thanks for your prompt reply.
        I’ve just tried opening the boot, but it doesn’t open. I can fully insert the emergency key into the lock and turn it clock and anti-clock wise, there is a ‘click’ sound. I can also fully push the boot button, but no luck opening it…
        I also remembered that car doors didn’t lock automatically when I was heading home yesterday.
        I also spoke to the previous car owner (he owned the car for a few years) but he said he didn’t have this problem…


      2. Hi Eve,
        Sounds like the air pump PSE that provides pressurised air to lock and unlock the door actuators has failed or for some reason is not running (it may be as simple as a fuse) the pump itself has failed or whatever is commanding it to operate is faulty. You should hear it running when you operate the locks. Try the open close button on the dash to see if that works as it should.
        All the best

  10. Hi Steve,
    Have just bought a 97 R170 SLK 230K and the key fob is faulty. Single button, flip out key.
    Using the blade it will unlock the drivers door and start the car. Central locking and alarm don’t seem to work via the fob or the key. Central locking arms via button on the dash, unsure about alarm operation from here, what is your advice please?

    1. Hi Oli, fault find the key fob as per the text, try synchronising the key from scratch, it will most likely be a faulty key fob, fob battery or fob switch that is preventing it working.

      1. Hi Steve,
        Only just realised you don’t mention the R170 model anywhere in the text. Must have assumed it was the same for all models. Is this right?
        Anyway, have replaced the batteries, the button on the circuit board and checked the board for faults and found nothing. Very strange!

  11. Hi
    How can I open the rear viwe mirror, where last month I found water inside so i think there is bad contact or dust in IR sensor inside.

  12. Thanks for the easy to follow instructions. My 1995 SL500 manual instructions did not work for me on my California spec US car. I’m all set. Great info. Thanks again

  13. Hi Steve,

    I have a 1992 300SL-24. I have tried the above fixes but with no joy. my key fob flashes once on the IR if i hold it down for more than a second or two but I do not have any lights on the rear view mirror.

    Any ideas?


    1. Sounds like the tactile PCB switch could be faulty, only making intermittent contact.
      Worth replacing, you can buy the switches from Ebay if you are handy with a fine soldering iron. Its like trying to solder ‘legs onto ants”!

  14. Changed batteries. Now the locks are locked and unlocked. Blade key will turn in the ignition and motor will turn over but won’t start and keep running. Another key and fob we have works to start the car and keep it running.

  15. Hi, if we replace the entire mirror with the IR receiver in it, is any programming needed (if mirror is new or used) or do we just have to sync it with current keys, does anyone know?

  16. R129 1996. When I use the door key to lock/open I do not get any red/green light flashing on the door handles. (When the fob worked I did get the light flashes).

    Should the lights flash on door etc when using the actual key?

  17. Can you simply swap a R129 homelink mirror ( 3 black rubber button on underside) with the rR129 Mirror that has the red/green light box attached to the underside?

    My R129 is a 96 manufacture but seems to have the 1997 rear view mirror.

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