Programming for new key FOB or not??!!
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Junior Member
Joined: Sep 2025
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2008 Mercedes C300
Programming for new key FOB or not??!!
So my sister used my car while I was out of town and when she came out of the store and proceeded to get in the car the key fob would not unlock the doors so her boyfriend broke into the car through the window of course then setting the alarm off. I believe she tried to then start the car but it would not start so at that point the battery was replaced in the remote. Not too sure why that wasn't the first thing done but I was not there. Since then the only thing that happens when the key is turned is the radio will come on but the car will not start. I dont know how often or what reasons that lead people to having to get new key FOBs but any suggestions on what can be checked out and eliminated as possibilities for the issue would be really helpful. ** Note: Key FOB does not have the slide in key for it.
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Joined: Oct 2012
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From: Hollywood, CA
W209, W124
Ok, gotta be scientific here.
1. Does the key work for locking/unlocking from a distance?
2. Does it work for cconvenience function? (Pointing at black square on door handle)
3. Did something happen to it? Sis may be embarrassed to admit it but it could really help to know right now. If it got dropped hard or got submerged in a liquid, or if lights got left on in car and battery went dead, VERY helpful to know now.
4. So key now turns in ignition but car doesn’t start? Are you sure it isn’t just a dead car battery? $10 meter can tell you, or maybe dash display.
1. Does the key work for locking/unlocking from a distance?
2. Does it work for cconvenience function? (Pointing at black square on door handle)
3. Did something happen to it? Sis may be embarrassed to admit it but it could really help to know right now. If it got dropped hard or got submerged in a liquid, or if lights got left on in car and battery went dead, VERY helpful to know now.
4. So key now turns in ignition but car doesn’t start? Are you sure it isn’t just a dead car battery? $10 meter can tell you, or maybe dash display.
Thread Starter
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2008 Mercedes C300
Because she has been trying to find a ride back and forth to the store where the car sits to try different things I will have to see if she can get back there to find out the answer to the #2 question. I have already asked her if the key FOB unlocks/locks the doors and she said yes.
It has not been dropped in liquid. As far as dropped hard can't say it hasn't hit the ground before.
As far as what's taken place the last 4 days from what I have been told regarding the vehicle battery is that there has been the attemp to jump the car several times. A multimeter was used to check the voltage of the baterry before (2.37v) and after (12.19) letting another vehicle try to give it juice. The flashers stayed on even until the next day she said. Another thing that we aretryong to figure out is if the car also has an auxiliary battery that needs to be checked.
It has not been dropped in liquid. As far as dropped hard can't say it hasn't hit the ground before.
As far as what's taken place the last 4 days from what I have been told regarding the vehicle battery is that there has been the attemp to jump the car several times. A multimeter was used to check the voltage of the baterry before (2.37v) and after (12.19) letting another vehicle try to give it juice. The flashers stayed on even until the next day she said. Another thing that we aretryong to figure out is if the car also has an auxiliary battery that needs to be checked.
Thread Starter
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2008 Mercedes C300
CORRECTION: After the battery was replaced in the FOB it did unlock the doors once but then stopped doing it which Im assuming would be because the vehicle battery voltage was getting lower.
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S65
Edit (we must have been posting at the same time. Sounds like the key fob battery died and the boyfriend proceeded to create a lot of issues with breaking into the car... because cars will start after you break into them if they can't be unlocked with the fob...? or some such thing.
Get the battery properly charged or replaced and hope all that jump starting didn't fry something.
Last edited by Sathinas; Sep 1, 2025 at 06:12 PM.
Super Member
Joined: Oct 2012
Posts: 538
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From: Hollywood, CA
W209, W124
12.19 won’t do. Ideally she gets battery charged and starts over.
There is some part of this story that you haven’t been told.
Odds of both batteries coincidentally dying at same time is ZERO.
Make sure they have key out of ignition while connecting battery or charger.
There is some part of this story that you haven’t been told.
Odds of both batteries coincidentally dying at same time is ZERO.
Make sure they have key out of ignition while connecting battery or charger.
Thread Starter
Junior Member
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2008 Mercedes C300
Apologies for being confused but just want to clearly understand when you say the two batteries are you referring to the one In key FOB and under the hood? Does the car have 2 batteries, a main one and auxiliary? Just asking because I read that the auxiliary battery has to do with the start/stop system in the car.
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Super Member
Joined: Oct 2012
Posts: 538
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From: Hollywood, CA
W209, W124
I don’t know if your car has 2 batteries.
I’m just saying that there is no way that the fob battery went dead, requiring that the car get broken in to. And then the car battery also went dead, so that nothing works now. Car would start up just fine with dead fob batteries. Key is designed so that it works in the ignition switch, even with no batteries in key.
Far more likely they just left the lights on and ran the battery down.
This is why I say you need to choke the truth out. There is just no believable way both batteries went dead at once, by coincidence.
I’m just saying that there is no way that the fob battery went dead, requiring that the car get broken in to. And then the car battery also went dead, so that nothing works now. Car would start up just fine with dead fob batteries. Key is designed so that it works in the ignition switch, even with no batteries in key.
Far more likely they just left the lights on and ran the battery down.
This is why I say you need to choke the truth out. There is just no believable way both batteries went dead at once, by coincidence.
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2021 GLB250 FWD, 2023 GLA250 FWD, 2013 C250, 2015 GLK350, 2005 ML350SE, 2003 E320
Lots of unanswered questions here and I’m not interested in affixing blame, only fixing the issue.
Some truths:
-As stated before, the car will start with dead fob batteries. The fob is inductively powered by the ignition switch.
-Leaving the lights on should not have run down the battery. There is a protective device that shuts off circuit draw when the engine is off to preserve enough electrical power to start the car. These cars are getting old, so I suppose that could have failed.
-If the window was broken, the antitheft system has been activated. The car has multiple anti drive away features including an engine/transmission immobilizer.
You may be in for a lift to the dealership to get that sorted out unless someone can post the sequence to reset the antitheft system on a W204. The sequence is simple and can be done from the driver seat, but I don’t have W204 documentation available any more. Before resorting to trailering to a dealer, try disconnecting and removing the battery, then fully charging it outside the car, then re-installing it.
Good luck. Report back here with additional information, questions, and hopefully resolution.
Some truths:
-As stated before, the car will start with dead fob batteries. The fob is inductively powered by the ignition switch.
-Leaving the lights on should not have run down the battery. There is a protective device that shuts off circuit draw when the engine is off to preserve enough electrical power to start the car. These cars are getting old, so I suppose that could have failed.
-If the window was broken, the antitheft system has been activated. The car has multiple anti drive away features including an engine/transmission immobilizer.
You may be in for a lift to the dealership to get that sorted out unless someone can post the sequence to reset the antitheft system on a W204. The sequence is simple and can be done from the driver seat, but I don’t have W204 documentation available any more. Before resorting to trailering to a dealer, try disconnecting and removing the battery, then fully charging it outside the car, then re-installing it.
Good luck. Report back here with additional information, questions, and hopefully resolution.
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From: Northern Indiana
2014 c300 w204.080 3.5 v6
Here's what grok had to say about resetting this.Resetting the Anti-Theft System on a Mercedes W204 (C-Class 2007-2015)
The Mercedes-Benz W204 C-Class features an advanced anti-theft system (often integrated with the immobilizer and COMAND infotainment) that can trigger due to a perceived security threat, such as a low battery, key fob issues, battery disconnection, or even a communication glitch between modules. This may display a message like “Anti-theft protection activated” on the COMAND screen, prevent the engine from starting, disable the radio, or lock out other functions. While the system is designed to deter theft, it can sometimes activate falsely.
Important Safety Notes:
Many triggers resolve with these non-invasive methods:
A weak vehicle battery (under the hood or in the trunk for some models) is a frequent culprit, as voltage drops below 11.5V can trigger the alarm.
The Mercedes-Benz W204 C-Class features an advanced anti-theft system (often integrated with the immobilizer and COMAND infotainment) that can trigger due to a perceived security threat, such as a low battery, key fob issues, battery disconnection, or even a communication glitch between modules. This may display a message like “Anti-theft protection activated” on the COMAND screen, prevent the engine from starting, disable the radio, or lock out other functions. While the system is designed to deter theft, it can sometimes activate falsely.
Important Safety Notes:
- Always ensure the vehicle is in a safe, parked position before attempting resets.
- If the car won’t start at all or the issue persists after these steps, it could indicate a deeper problem like a faulty Electronic Ignition Switch (EIS), damaged wiring, or the need for key reprogramming. In such cases, consult a Mercedes-Benz dealer or use a diagnostic tool like Xentry/Star Diagnosis to scan for codes (e.g., B1561 or 9561). Do not force-start the vehicle, as this could damage components.
- These steps are based on common fixes for W204 models; refer to your owner’s manual for model-specific variations.
Many triggers resolve with these non-invasive methods:
- Verify the Key Fob and Battery:
- Replace the key fob battery if it’s low (CR2025 type for most W204 fobs). A weak fob can cause the system to misread the immobilizer chip.
- Test with a spare key if available. Insert the key into the ignition slot (if equipped) and try starting.
- Lock and unlock the doors multiple times using the fob (press lock, wait 5 seconds, unlock). Get in, close doors, and attempt to start.
- Reset the COMAND System (If Message Appears on Screen):
- With the ignition off, press and hold the COMAND power button (usually on the head unit) for 10-15 seconds until the screen restarts or goes black, then releases.
- Turn the ignition to position 1 (accessory mode, no engine start) and wait 1-2 minutes. Turn off and try starting.
- Alternative: Turn the ignition to position 2 (on, dash lights active), press the fob’s lock button until doors lock, then turn ignition off to 0. Unlock with fob and try starting.
- Door Lock Reset Trick (For Immobilizer Sync):
- Insert the key into the driver’s door lock cylinder.
- Turn to unlock, then lock, then unlock again (cycle 3-5 times quickly).
- Remove the key, unlock with the fob, enter the car, and attempt to start within 30 seconds. This can resync the system if triggered by door sensors.
A weak vehicle battery (under the hood or in the trunk for some models) is a frequent culprit, as voltage drops below 11.5V can trigger the alarm.
- Charge or Jump-Start the Battery:
- Use a trickle charger or jumper cables to bring the battery to full charge. Let it charge for 30-60 minutes with the ignition off.
- After charging, lock the car with the fob, wait 5 minutes, unlock, and try starting.
- Disconnect and Reconnect Battery (Hard Reset):
- Caution: This will reset other systems (e.g., clock, windows need recalibration). Park on level ground, turn off ignition, remove key, and wait 10 minutes.
- Open the hood (or trunk for battery location on your W204 variant).
- Disconnect the negative (-) battery terminal first (using a wrench), then the positive (+) if needed. Touch the terminals together (with cables disconnected) for 5-10 minutes to discharge residual power.
- Reconnect positive first, then negative. Tighten securely.
- Insert key, turn to position 1, wait 90 seconds (or up to 10 minutes for full sync), then try starting.
- If the radio locks, you’ll need a 5-digit radio code (found in owner’s manual, glovebox card, or via dealer with VIN).
- Check Fuses and Sensors: Inspect the fuse box (under hood or dash) for blown fuses related to the alarm/EIS (e.g., fuse #15 or #32—check manual). Look for damaged door sensors, hood latch, or wiring harness under the dash.
- Wait It Out: Some systems auto-reset after 15-30 minutes of no activity. Lock the car, wait, then unlock and start.
- Professional Help:
- Visit a dealer for reprogramming (costs vary; they use Star tools to enter a 25-character PIN linked to your VIN).
- Use an OBD-II scanner (e.g., YOUCANIC or iCarsoft) to read/clear codes. If code B1561 appears, it often needs dealer-level reset.
- Avoid disconnecting the battery repeatedly, as it can worsen the issue by erasing security data.
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From: Hollywood, CA
W209, W124
As with most AI, that is nearly useless and not much help. It keeps referencing using the key in driver’s door. That isn’t going to help as it is only the metal key, which just sets the alarm off. It appears to be mixing advice for different years into a soup.
And I wasn’t trying to find someone to blame. I’m pointing out that both batteries didn’t die at same time. Whether by omitted action or embarrassing mistake, the car has been turned into a brick. The quickest, and best route back to drive able might get obscured by someone trying to avoid blame.
You just want to get the truth so you can get it running as quickly and cheaply as possible. I doubt you need a new key or programming, just trying to help you avoid a $1,000 dealer visit over a $100 issue.
And I wasn’t trying to find someone to blame. I’m pointing out that both batteries didn’t die at same time. Whether by omitted action or embarrassing mistake, the car has been turned into a brick. The quickest, and best route back to drive able might get obscured by someone trying to avoid blame.
You just want to get the truth so you can get it running as quickly and cheaply as possible. I doubt you need a new key or programming, just trying to help you avoid a $1,000 dealer visit over a $100 issue.
Last edited by MacVidMB-V8; Sep 2, 2025 at 12:46 PM.
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Try taking out the battery, charging, and then reinstalling it. The security and drive authorization systems are electronic, not mechanical. The only way this would not work is if there are nonvolatile memory chips storing the incident.
If someone reading this has a W204 training manual, the reset from the driver seat is in there.
Apology to @MacVidMB-V8 but I’ve had relatives borrow my MBs and create myster disasters to which they will only provide misleading information.
If someone reading this has a W204 training manual, the reset from the driver seat is in there.
Apology to @MacVidMB-V8 but I’ve had relatives borrow my MBs and create myster disasters to which they will only provide misleading information.
Last edited by Odd Piggy; Sep 2, 2025 at 10:05 PM.
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A trip to the dealer may be necessary (and ultimately cheaper). Jump starting these cars can be tricky and should be avoided if at all possible. If a starter battery is dead (<10.8V), it's best to replace it with a new battery or remove it and try recharging out of the car.
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2008 Mercedes C300
So here's a status update a locksmith was called out to the vehicle and he did a Diagnostics run through on the vehicle first before trying to do a key fob. He said there is no codes that came up for the EIS or the ESL modules. However, there were some that came up for the front and rear SAM.
1. He said the key fob showed that it had no data to it which is not so much speaking of the programming data he's just saying that most the time you can see how many times the keys been used to start the car etc. But that mine shows nothing so I need a key FOB. When I ask him why a message wouldn't come up saying that the key is not recognized if it has no data he could not answer that. He said unless the system was just sleeping. 🤷♀️
2. Every time the battery is connected the alarm continues to sound and I asked him if that was normal or if that was an indication that something needed to be handled with the anti-theft part of the car due to break it into it and then having all the issues. He said that the alarm was going to continue to sound until a working key was put into the ignition. Although, if I knew what the codes the SAM module threw I could possible tried to get those resolved.
I am going to try the steps for resetting the anti-theft system through the COMAND and see what that does.
1. He said the key fob showed that it had no data to it which is not so much speaking of the programming data he's just saying that most the time you can see how many times the keys been used to start the car etc. But that mine shows nothing so I need a key FOB. When I ask him why a message wouldn't come up saying that the key is not recognized if it has no data he could not answer that. He said unless the system was just sleeping. 🤷♀️
2. Every time the battery is connected the alarm continues to sound and I asked him if that was normal or if that was an indication that something needed to be handled with the anti-theft part of the car due to break it into it and then having all the issues. He said that the alarm was going to continue to sound until a working key was put into the ignition. Although, if I knew what the codes the SAM module threw I could possible tried to get those resolved.
I am going to try the steps for resetting the anti-theft system through the COMAND and see what that does.
