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Won't start...somtimes

I have a 2003 Infiniti g35 with about 140,000 miles on it. Its been having a strange problem recently and intermittently. Sometimes when I first get in the car and I put the key in and try to start it NOTHING happens. I mean nothing. Not sound, no lights anything. If I slam the door and try the key again the dash lights may flick on for a second but then nothing. I’ll open the hood for a bit and slam it and try starting it again and it will usually start again fine. It also seems to happen more when its hot outside or the car has been sitting in the sun. Other things notable about this problem is that MOST of the time, things like the radio presets are unaffected. That is, when the car does start it’s not like it was a cable from the battery or anything since the radio was still getting power. Another, possibly related problem, is if the car is running and I park, but have not turned the ignition off yet because I’m listening to the radio (those driveway moments) and reach up like I’m ready to turn the car off and just ever so slightly grip the key, the radio will die and the engine will keep going. These things lead me to believe that it all has something to do with the actual ignition switch where the key goes in. Of course when I drop it off at the mechanic it does not get reproduced and he thought it was the starter until I reiterated the fact that NOTHING happens when the key turns. It can’t be the starter since when you turn the key there is no power to anything. This has been going on for over a month but I finally got fed up when I couldn’t get it started after stopping at the 7-11 on the way to work. I took it right into the shop but the mechanics opinion is “if I can’t get it to act up, I can’t diagnose the problem” which seems kinda lame to me, but what can I do.

When this happens, does the dome light turn on like normal with the door open?
And if so, does that light die when you turn the key?

If it doesn’t, your ignition switch may be at fault. It isn’t likely to be a starter or interlock because you say you don’t see dash lights when you turn the key.
If it does, your battery or a connection to it may be at fault.

No dome light or anything. The key just turns in the ignition but absolutely nothing happens. Is there a way the mechanic can test this without having to show him that it is currently failing?

That’s pointing in the direction of the battery or a connection to it. The dome light draws very little current and is usually a pretty simple circuit. Battery juice must not be getting to it for whatever reason.

Do you know how to use a volt meter?
Get a cheapie one at Harbor Freight or Sears. Set it to read ‘DC volts’ and be sure you understand what terminals go where to measure voltage in the 12V range (it will most likely be the 20V range).
Next time it happens, measure across the battery. It should read 12V.
If it doesn’t, put the negative lead on a large bare part of the engine and the positive lead on the positive battery terminal. You should read 12V.
If it does not, you have a bad ground cable somewhere.
We can go on but let’s first try ^that.