Just thought I would post to explain about a somewhat common issue people are getting on their W203s and how it was resolved for me. This may not be the same problem you have, so I will explain in detail.

I had the issue of the key not being recognized/not unlocking the steering or any reaction whatsoever when I would put it in, but only after the battery had died (drained overnight)

I am a service technician (mechanic if you prefer), and somewhat familiar with MB systems.

This has happened before, once when the battery had died, so I checked and noticed the battery was drained (known seat module drain, repaired but has reoccured about 1 year later, usually left unplugged but we had forgotten to unplug it this time).

Tried a regular boost. Didn't help (unlike the time before)
Tried fully recharging with a trickle charger and tested with sophisticated battery testing equipment, battery fails test.

Replace battery. Does not improve situation BUT now battery is KNOWN good.

Checking fuses. All ok in all fuse boxes.
Key will unlock doors etc, but not recognized when inserted in the ignition switch (aka EIS)

Send car to MB. dealer.
Technicians say the key is faulty, and to start from there, but nothing is assured. I only have one key (original key, light physical damage)

Replace key at MB. (250$+50$ for the mechanical key) + 1.5hr diagnostic/programming.

Vehicle fully functional with new key.


I still don't understand why/how changing the key fixed the issue, since it only occurred after the battery died, I'm not even gonna guess.

Long story short: Replaced key and battery. All good.

Even my RF unlocking/locking is fully functional (for now), instead of intermittent/nonfunctional.

edit: canadian car/price of key

 

Unfortunate that you paid "programming" costs. The keys come programmed when they are shipped.

It sounds like your key simply crapped out. I guess that could happen after a long period of time.

 

anyone with an EIS merc should always have two keys, they usually give no faults before they fail, usually there is nothing physically wrong with the key itself, a hash error occours in the eis and the calculation that is done in the key and eis do not match anymore so the key refuses to turn.

The dealer just has a new key programmed and all is ok.

Low battery voltage can cause the weirdest of things to happen to electronic system, data can easily be corrupted and cause nasty things to happen.

 

Straight up. Now I have the lovely task of changing wheel bearings, compression arms, rear brakes... and I'm not even gonna bother with the squeaky bushings yet. Very depressing car.

 

How is it depressing it is an old car these things happen. I am surprised you have bad wheel bearings mine are perfect still at 175,000 Miles. What are compression arms?

 

I guess that could happen after a long period of time.

 

you lucked out there. I had to buy a new key AND pay the $1400 repair for a new EIS

 

Glad I have both original keys to mine. Every time I see the new key design I want it, but I just can't justify it when both my originals are in perfect condition.

 

It is much prettier, though. Have to say that I'm glad I got the newest style one for my normal key. Still have the original one for my extra, but damn, are the ugly.

One added benefit is that the hatch open button is an actual button - on the original key, you could depress the area where the button should be, but it's a very annoying thing to do. So at least on "coupes", the new key does make some sense.

 

I feel your wallet pain too.. I'm having a problem when I turn the fogs and headlights on, push the key in turn it once all is good, turn it to position two (before start) you hear all the stuff moving the fuel gauge goes up and then all of a sudden the interior lights brighten up fuel gauge drops back down and everything is like I didn't turn it. And if from there I attempt to start the car the key won't turn [right] however it will turn left for me to remove it out. It starts if I keep all lights off and start it. I have a new car battery so I am unsure if it has to do with my key cause mine is badly beaten up. My alternator works fine, car registers an internal voltage of 14V when started and 12.5V when off. I may have to do the same but damn MB Canada charges so much for a new key and for it to be programmed.

Note: I usually turn all might lights on first to avoid the error messages on the dash, I haven't done xenon=present yet. = $$$.$$ .

 

Unfortunately my car came with aftermarket wheels/spacers/hub adapters and it doesn't take long to destroy wheel bearings/ball joints (compression arm is one of the control arms that has a ball joint in it) compared to OEM wheels.

Not to mention the rust. My 97 civic has less rust.

@xijonix my car was doing weird things before too. It would take a second or two before the interior lights would come on, sometimes they were dim, i would get a "light sensor malfunction" randomly and my RF for the remote was highly intermittent with no pattern at all. With the new key the car is (electronically at least) perfect, minus the seat module draining.

 

That's amazing, my problem that day actually got worst the key no longer worked and when I purchased my vehicle I only got one key. So that night I took the key back portion out where the battery lays removed the tray and used a can of compressed air and blew the crap out of it, saw a bunch of dust just flew out. Then i took the same can of compressed air and blew inside the EIS around the area that turns. Gave it about 10 minutes for any big of fog to disappear. Pushed my key in and it worked perfectly like new! I was so damn happy, before when I pushed my key in lightly the EIS won't detect the key I would have to push it in hard. Now I can lightly push the key in and it detects it right away. However tomorrow I'm going to the dealer to order a new key I don't want to risk it again. I may purchase a new housing for my current key, as it still functions and would be good as a spare if anything. Thanks a lot for your tips, narrowed it down to the key. I thought it was a bad EIS, that would be $$$.

 

My car is two weeks old and I love it. But yesterday,a problem started involving my key. I noticed in other posts people couldn't get the car to unlock. Mine is the opposite problem. I cannot get the car to lock using the dimples on the handle. I must use the key. Anyone else with this problem? Any possible solutions?

 

You are in the w203 forums try the w205 forums if you have a 2015 c400.
As for your issue I would try your 2nd key fob.
When you take it to the dealer bring both keys they will need it.

 

My 1998 e300TD has this key issue. However, this car opens and closes the locks, and the shifter goes into any gear I put her in. However, the key simply does not turn in the ignition cylinder. Could this be a battery problem, key problem, ignition cylinder problem or all three

 

So I took out the motherboard ( there are videos for that on YouTube) and look at what I found lint blocking the infrared sensor

Lint blocking infared sensor