Greetings: 2008 S4 cab with only 37,000 miles. Vehicle hard to start on occasion but lately stalled out while running and could not restart. Took to shop and had one bad cell in battery and numerous error codes. Replaced battery and error codes disappeared. Got car back and drove for about 15-20 minutes then stalled out again and would not restart. Waited a couple of hours and tried to restart and after a hard crank restarted. Suggestions or ideas what the issue could be? TIA

 

FYI - it turned out to be a crankshaft sensor

 

A faulty fuel pump or incorrect timing can cause this. If the engine is over-fuelling, that can cause it to stall. More commonly known as flooding, over-fuelling wets the spark plugs and they aren't able to ignite the fuel in the cylinder. It causes a misfire that can cause the engine to stall. Check this out: https://docsbay.net/what-causes-a-car-to-stall

 

Crankshaft sensor was my first guess too.
I had it happen in my car. 210,000kms. A cheap OBD2 unit turned up the p number for it.
My sensor unit went open circuit at 40+ °C. I tested it in hot water after removing it.
So thats the reason why it wont wont start when hot. It runs bad, wont idle properly when it is going & my car popped a CEL light.
A cheapish easy fix, 1 bolt + 1 plug. My replacement unit was a generic US$30 part.
The broken one wasnt original I think. Bosch
Original part was US$135.
I expect in the USA it would be US$100 or less.