My car was running great when all of a sudden it just quit with no warning, so we replaced the ECU and now we don't have any fire to it, can you tell me how to check and see if the ECU is bad or something else going on?

What kind of car is it? It could be a sensor like cam sensor or crank sensor. On jeeps they had a flywheel sensor. All of those told the ecu when to fire the spark plugs.