Hi all got a 1.4 2001 polo that will not start when warm. It starts from cold no issues but turn it off and on a few times and it wont start, it does not kick over. I thought it was the solenoid in the started but have replaced the starter motor and it still happens. I have tested the battery when running and it is charging and have swapped the battery for another one I know is fine, but this has not helped. The headlights go off when the key is turned so I dont think it is the ignition but have run out of ideas as to what is causing this, it bump starts without any problems and it is only when it has been running so at a loss. Any one any ideas what to try next.

Battery positive to starter, and then alternator. Check those, check the engine earths and battery negative to body.

Whats the alternator charging at? Whats the battery's voltage when standing?

Alternator charging about 13-14 and battery12.7 when standing but why only when the car is warm?

Faulty temperature sensor?

Not had a chance to test anything else as yet but the car started fine is morning for work but would not start after work, got a friend to try jump starting but didnt help, bump started with no problems!! Have got a friend to bring his diag stuff to work tomorrow so will update with anything we find. Am thinking it might be the ignition relay but that is just a punt, hope to resolve tomorrow.

Did a bit of testing and have diagnosed that the switch in the ignition is not working properly (no voltage down the red and black lead when key in start position and not starting, voltage down the lead when it does start) have considered replacing the switch but this looks like a hassle and my mate has a spare start button so going to by pass the switch and pretend I have a posh car that has a start button.