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.