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I now know that the Mercedes EIS and smart keys are notorious for problems but I am still searching for a resolution to mine (1999 C280). I have experienced the problem of the key not turning - it can take 3 to 10 tries of inserting key without it being able to turn, removing it, and trying again. So far, it eventually does turn (can take 5 to 10 minutes of tinkering) but I worry about the day it doesn't. My mechanic did a scan and did find the EIS as a problem. We understand that the recoding that occurs with the infrared keys can go bad. I have had the ignition module replaced (didn't solve the problem) and bought a new key (that didn't fix it either). Two patterns show up with this problem: it's almost always a problem on the first start of the day (after running the car and using it during the day it seems to be fine) and it's worse as the overnight temperature is colder.

What other system in the car could be affecting the key coding system? Any ideas on what to look into are appreciated.
 

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You need to run the codes and observe the actual values of the system on a MB STAR tester to properly ascertain what the problem is. You need a dealer to diagnose this (or a VERY well funded indy, those machines are not cheap). So far it sounds like a logical chain of repairs, but proper diag would have helped you avoid spending the money on parts that didn't fix the problem. The indies are a gamble. I'd let them change my brake pads but wouldn't look to them too much to diagnose more complex problems.
 

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I believe my shop used the proper equipment to scan. I have the print out and it did show 2 codes for the electronic ignition switch with text that read "The control unit has undervoltage". That information is what led to the EIS replacement and when that didn't solve the problem we load tested the battery to be sure that wasn't the source. Because it occurs after sitting overnight and is worse in colder temps, I can't help but think it has something to do with power draining. But everything else works fine and the scan didn't identify anything that seemed related to this problem. It's just the key that is very slow to code and respond in the mornings. Today when it was below 25F, I spent 10 minutes trying and then just left the key in the ignition for 10 minutes. It finally engaged.

Any other thoughts? I appreciate any ideas with winter fast approaching.
 

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Just for grins, try hooking up a battery maintainier overnight while the car sits. Undervoltage will cause all sorts of weird little faults. Eliminate the battery completely from the equation and we will be able to more accurately ascertain the cause of your problem.
 
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