I've got an 01 civic that was running fine and after it sat for about 1 week it doesn't start. Battery is good and cranks normal. I checked for spark already and no spark. There was a check engine light on with a P0420 code but i hear it's not related.

Any suggestions on what it can be?

I've heard main relay or replace ecm but i don't want to throw parts at it without getting some more feedback

 

Correct, P0420 is catalytic converter related. It might mess with your fuel ecomony but not cause the can to fail to start. When you turn the key to "ON" (II) do you hear the fuel pump in the back of the car and the main relay in the front?

Main relay is super cheap, but if you can crank normal that's not the problem, as the crank would not happen without a main relay.

Also might want to do a compression check.

Lastly, maybe it's flooded? unplug your injectors and try to start. Friend's integra did the same thing every now and then.