99 e320 w210 key won't turn in ignition
 

The electronic key is not being recognized by the infrared key system in the 210. It doesn't engage with that click when you insert the key. And of course the key won't turn. I suppose it is either the electronic ignition key unit/optical system or the steering lock. Or its just a bad key. I changed the battery and the battery store tested it, fwiw. Any thoughts on that?

The coin battery has nothing to do with starting the car, the igntion key module inductively charges the key when its inserted.

It is empirical that the battery has ample voltage to start the car otherwise the key may not work. If its really cold where you are, first charge up the battery with another car and use a hairdryer on the ignition key module then retry.

Alas, I'm in Las Vegas. 52 F today. You are correct about the voltage. It could be that simple. I will attach a charger before anything else.

Just recently had this happen on the 02 E320 that I fixed up for daughter. Problem was the steering lock mechanism p/n 208-462-04-30.

I just reviewed this gadget on Amazon even though I didn't need it .... yet.
Subsequently, I had an opportunity to work on another family E320 and it helped
narrow down the troubleshooting options and saved a little bit of tail chasing.
The culprit turned out to be a break in the coil and a small dab of soldering did
the trick.

This tool is pretty neat - I'll buy it just because its so easy, A freind of mine made me a similar item but its a simple LED and a circuit that inductively fires it - Need a bit of darkness to see it.

If the induction coil is not working, the key wont spit the code to the ignition.