Intermittent fault
 

I have a 2013 Honda Civiconce in a while it wil not start. This happens if I have been using it, switch off the engine and then try to start it again, for example to pop into the shop. If I try again in about 1/2 hour it wil start. This is making me anxious about switching the engine off if i know i will need to restart it soon even though it isn't every time and i also can't use start/stop as a result. It has been to the Honda dealership but the diagnostic is coming up with no faults and they don't know what is causing it. Has anyone had this problem or know what might be causing it?

welcome to the forum. Key ignition switch if you carry a lot of keys on the same key ring as the car key. main relay, fuel injectors

Sometimes the cold solder joints on the main relay. Retouch with hot soldering iron and a little flux
Other possible causes are faulty fuel pump, ignition switch, crankshaft position sensor,
cam sensor, park/neutral safety switch, corrosion on battery terminals just to name a few things.

Personally, I would clean up the battery posts and cable terminals.
Follow negative cable and clean chassis ground. That cost no money and just time for DIYers.
Then if it does happen again, shift the gear selector thru the gears and back to neutral. Try starting.
I had this intermittent crank, no start. I ended up cleaning everything and replacing the ignition switch.
Never had the problem again until I junked the car. Too much rust.

When crank, no start, remember that the engine requires air, fuel, spark and compression to happen in the sequence
Also, many videos on youtube on "crank, no start". Can help in case it happens again. You will have some knowledge.
Good luck.