My Honda Civic will not start when it is hot inside the car- why?
It’s most likely your fuel pump relay, located just behind your dash near the floor on the driver’s side. You can likely replace it yourself; I did with mine and it worked like a charm
What are the symptoms? Does it crank?
Make mine another vote for the fuel pump relay. This is a common problem on older Hondas. The relays fail under high heat conditions when they get old. A service manual, such as Haynes, will show you the location of the relay.
You remove the old one and plug in a new one. Problem solved.
As Goldwing implied, if the engine cranks but does not actually start, then the Main Relay underneath the dashboard is the likely culprit.
We don’t know the model year, the odometer mileage, or the maintenance history of this mystery Civic, but in the absence of all that information, Honda’s classic Main Relay problems do come to mind as the cause of this no-start problem.