Car will sometimes start and sometimes not

Lately when i sit in my car while it’s running for maybe 20-30mins i’ll turn it off and when i got back to start it, it won’t at all, it wont even make a cranking noise, maybe once. the battery is fine in it because all of my lights will turn on. anytime this has happened which it’s happened twice i’ve let the car just sit a day. the next day i’ll come to my car wherever it’s parked at and it will start just fine. now if i turn my car off it won’t start unless i just let it sit for so long and then it will start just fine. any ideas or help? it’s a 2011 Volkswagen jetta 2.0 TDI

How old is the battery? Could be that or some starter issue. I would go to an independant mechanic to get the charging system inspected because its no fun getting stranded in the middle of nowhere

-check your battery cables and make sure they are clean and tight. no green stuff, and you shouldn’t be able to move the cable where it attaches to the battery.

-Next time it won’t start, move your gear selector into neutral and try to start (assuming this is an automatic transmission.)

-don’t assume your battery is good- actually get it checked. It takes a lot more effort (and thus more ‘juice’) to turn over an engine than it does to turn on lights.

when it started this morning i drove it to Autozone to get the starter tested, when i had to turn the car back on to test, the car wouldn’t start, he went ahead and tested the battery and he said the battery was fine. maybe the alternator or starter he thought. the battery is probably less then a year old

battery is relatively new

That would have been to perfect time to verify that the starter is failing, I guess they don’t know how to diagnose starter issues.

When the engine won’t crank, measure the voltage supply at the starter battery cable and the solenoid connector.

I ended up getting it towed to a local mechanic that i went to get my engine actually fully replaced, hoping its just a starter issue. I didn’t even think to test the alternator or battery cables

Just to clarify…
Is this the same VW TDI whose engine you had to replace less than 1 year ago?

yes sir. has been running great until lately.

let me rephrase that, it runs great! just a starting issue it seems

A bad alternator wouldn’t cause this. A bad alternator would cause the battery to go dead and that apparently hasn’t happened.

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I think your mechanic should be able to quickly diagnose this if the problem is quite repeatable. Or, at least eliminate some variables quickly if this is something oddball. While I agree it is not the alternator, it could be the ignition switch, cables, starter, ecu or a bad sensor.

I suspect a bad crankshaft position sensor. Dead cheap and easy to replace, so nothing lost if that’s not the issue, but sounds very much like what I’ve experienced on 2 different vehicles.