W204 C320CDI Key Problem

Hi everyone, I'm new to the site but hope someone can shed some light on this fault or point me in the right direction.
I have just purchased a 2007/57 C320CDi 7G that came with 2 keys. The seller advised me that only one key was working. The faulty key is the one that had been used since new until 2 weeks ago, the failure is as follows.
Key locks/unlocks the car, goes in the ignition, turn the key and the engine turns over but the car fails to start. It's almost as though the immobiliser is still activated. The spare key that I'm using now works perfectly but I dont want to wait until I have two faulty keys.
Any help would be appreciated.
I'm an Industrial Air Compressor Engineer based in Scotland, been doing it for over 20 years and if anyone needs any help feel free to ask and I'll do what I can.

 

Have you tried replacing the batteries in the key that doesn't start the car?
It's quite common for it to unlock the car but not disable the immoboliser (the engine will still turn over, but not start).

Otherwise there's Markrrp (or something similar) a forum member who runs a business repairing keys.

 

Thanks for the advice. I have not replaced the battery in the key yet but I'll give it a go.

 

The batteries only power the key to unlock/lock the car.

 

Thanks for your input.

Now I'm confused

Any ideas why the car starts with one key but not the other then? The engine turns over with both but only starts with one.

 

Erm.... The key is faulty???

 

The batteries do not have an effect on this part of the key

 

WG M-B, how about a solution then?

 

You can either purchase a new key from mb, or send the key to 'replacementmercedeskeys'. Mark is a forum member here and can repair the keys