Won't start when hot outside
 

Accord wont start when it has been siting in the heat for awhile. Main Relay needs to be replaced right?

Yup

My old 91 accord had this happen before.

I pulled the relay and re-soldered the connections on the board, popped it back in, and no probem until I retired the car a couple years later.

Main relays aren't that expensive, but if you have a soldering iron and a 10 milli deep-well socket, you may as well try the free method.

If, when you turn the ignition to the ON position, the check engine light comes on then goes off in a few seconds and you don't hear a "click" from behind the drivers dash at the same moment, I would go ahead and replace the main relay.

However, a main relay can still "click" and cause a no-start condition. Sometimes it is best to pay a competent technician to diagnose the vehicle. It can save alot of time and money.