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2002 C320 Wagon
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If they key starts the car just fine, but wont work for Remote Control Locking, then it's either (1) a weak or bad battery in the key or (2) a bad key or (3) a problem with the antenna amp under the rear glass. If they key also is not recognized by the ignition switch, then first try another key. If that works, then you have a dead key which will need to be replaced. If the other key doesn't work either, then it coudl be a problem with the vehicle battery, or the vehicle's security system (DAS/EZS).
If there is an issue with the antenna amp under the rear glass, what exactly would the problem be and how would you fix it? For me it's gotta be that because my infrared signal works most of the time to unlock the doors and I can always start the ignition just fine.
 

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Discussion Starter #22
@kentstjohn: When you cannot remotely unlock your car, can you unlock the door with the mechanical key and turn the ignition?
 

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2002 C320 Wagon
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@kentstjohn: When you cannot remotely unlock your car, can you unlock the door with the mechanical key and turn the ignition?
Yep. Mechanical key works everytime. The only thing that is intermittent is using the infrared to unlock the doors. Unlocking remotely does not work at all.
 

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Discussion Starter #24
i guess your assessment is correct and you're having a non-ignition-related problem. Good luck.
 

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Coupe/Convertible Forums Moderator
CURRENT: 2011 SL550 FORMER: C300, ML350, CLK550 Cabriolet, C240, ML320, 300TD
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The antenna amp sits right under the stubby antenna on the back of the car. On the sedans, it's not an easy job because it requires dropping the headliner. On the coupe and wagon, it's under the trim around the back glass. Sometime the problem is just that the contacts from the module that mate to the antenna filament wiring on the glass are dirty and spraying some contact cleaner under the amp fixes it. If you have to replace the amp, it's expensive (nearly $1K I think) and it's difficult to reattach to the glass. Most people just have the whole rear glass replaced with OEM that has the antenna and amp pre-installed.
 

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Hello, newbie here to the Mercedes world. Ive' always been a Audi guy so I am familiar with the German engineering but have to say wow to the electrical system on my recently purchased 01 c320. This morning when my son attempted to unlock his car the locks would not unlock with the remote. He did not try the IR window because he was running late and didnt think of it but he did mention the trunk would open and he was able to hit the lock button and the lights would blink as if it were attmpting to re-lock. Does this sould like the Amp or the EIS to you folks. thank you in advance
 

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Coupe/Convertible Forums Moderator
CURRENT: 2011 SL550 FORMER: C300, ML350, CLK550 Cabriolet, C240, ML320, 300TD
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If he was able to use the SmartKey RCL function to open the trunk and lock the car, then the antenna amp and EIS are working properly. It's probably the button on the SmartKey itself. I find that sometimes you have to hold it down for just a moment longer than you'd expect for it to react. It could also be a dirty contact or just a weak battery in the SmartKey. When you open the battery compartment, you can use tweezers to (gently!) pull the circuit board out and check the contacts.
 

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2002 C320 Wagon
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18 Posts
Thought I would post an update on what my issue was.

It was simply a faulty fuse. My mechanic replaced the fuse while doing some other work and the remote key works perfectly. It was that simple. No issues with antenna/amp, etc.

Hope this helps some folks who may be experiencing the same issue.
 

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2006 E350
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Hey, I am having the same issue with my 2006 e350, I have two keys, sometimes one of them work, sometimes both and sometimes none, the screen shows message with a red battery and saying visit workshop, maybe this is the problem after all, I will go to some workshop and give you a feedback, ..... and just to be clear I did not change the batteries of the keys yet.
 

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c240 2003
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Maybe solved

I have experienced the same trouble with the randomly working fob on my C240 2003. Although the ignition works fine when I may have successfully opened the door with the Physical key. I discovered that pressing the door unlock and trunk unlock buttons at the same time tends to eradicate the issue whenever it arises. Hope this helps.
 
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