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Discussion Starter #1
I just purchased a 2004 S500 that came with 2 nec type keys. One works perfectly. The second one would only work the doors and not unlock the ignition to start the car. The seller said it was that way when he bought the car. Today I took the electronics out of the case and made sure the case was clean and the optics were not being blocked. When I was testing the cleaned fob I found it remained the way it had been, not letting you turn the fob, but I then pushed the door unlock button and that opened the key module and the fob turned and started the car. I found that you can press any of the 4 buttons and then turn ignition on. Anyone know anything about this? Is there a feature in the security system that would require this extra action or do I just have a weird key fob?
 

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2001 210.082 +494, 2008 221.176 +494
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That sounds like a defective circuit board in the key, or a cold solder joint on the key circuit board.
 

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Discussion Starter #4
Thanks for that idea. I will look for a cold solder connection, I can fix that if I find it. I wanted to ask around if this could be a program issue before getting physical with the key's electrical stuff. I am good with electronics but dumb about This key fob. I see what looks like a miniature CT coil and diode at the tip of the circuit board, thinking this what the boards power supply is for the ignition. So if it is not working then no power for the circuitry until I push a button and then power from the batteries?? Just a theory. I will look close for bad solder.
 

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You are correct- the coil powers the encryption circuitry and enough power through induction to start the vehicle with no batteries installed. A test would be to remove the batteries and repeat your test. The induction does not supply enough power to (and likely does not have any physical connection) to the Remote Keyless Entry (RKE) circuit. Technically, the buttons should not invoke any function when in the EIS (Lock/Unlock is disabled).
 
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