'01 e430 - both keys sometimes don't start car
 

My keys are not working correctly. Every so often, one or the other won't lock / unlock the car for a few attempts.

Every so often, the ignition accepts the key, things light up, but when turned to start, the engine does nothing. If I try the other key, it usually works.

Today, neither worked for a few attempts. Then on the last try, I put one of the keys in, turned it, and left it turned. After a few seconds, the car started up. I started the car a few more times today no problem.

I've changed batteries, with no change in symptoms.

What can I check, clean, or generally fiddle with to help solve this problem?

I'd start with a NEW "extra" key..Possibly both keys are having issues.

Making sure that the red window on the end of the key is CLEAN is also important.

Just curious - where's the antenna or sensor that receives the signal from outside the car?

Both keys are clean on the ends - can I clean the ignition switch receiver with a q-tip and alcohol?