Hello, I've been working on this issue for about a month now and I'm really not sure where to go from here, maybe someone can point me in the right direction?

Issue: For the last month or so, car occasionally won't start when I turn the key. I get one 'click' sound and then nothing. There is no engine cranking sound and no 'click click click click'. I initially thought this was a problem with the neutral safety switch, as sometimes the car would start if I put it into neutral. However, yesterday my car failed to start and would not start no matter what I did, even in neutral. The car eventually did start (after about 20 attempts), after I jiggled the keys. I don't know if that was a coincidence or not. Since then, the car has started normally.

Diagnosis so far: I have cleaned the battery terminals and tested the battery, starter, and alternator. All appear to be working normally.

What next? The way I see it, there are 4 possible causes. 1) The ignition switch, 2) the start relay, 3) the neutral safety switch, or 4) some loose connection anywhere in the ignition system.

Does anyone have recommendations on how I should proceed in diagnosing this issue? Similar experiences or insight?

Thanks in advance.

That's almost certainly bad brushes in the starter motor. The solid click you're hearing is the solenoid pulling in, but the juice isn't making it to the starter armature. To prove it, next time it misbehaves, smack the starter with a heavy hammer or hand sledge while an assistant holds the key in the Start position (on a Sentra you'll probably need an intermediary tool like a metal pipe or long prybar). If rattling its cage wakes it up, the brushes are worn out and losing contact with the commutators in the armature. That will be a come-and-go occurrence, some of the brushes and commutators are bound to be less worn than others. Whenever it lands on a bad one, you're getting a no-crank.

Oh wow that wasn't even really on my radar! Thank you so much I will definitely try giving the starter a whack next time and seeing what happens.

You're most welcome. Post back and let us all know how it goes.

So I went ahead and replaced the starter. Out of curiosity, I took the old starter apart to check the brushes and... it seems like they were fine? I'm attaching a photo, would love if someone can give me their opinion on it.

Judging by the spring positions, brush #5 looks trashed (or the spring is). The rest look okay.