Black screen on bootup, sometimes flickers

I have a iMac with what seems like a faulty display. When I boot on the computer I get the typical mac bong sound but I do not get a display or it would come on flickering. The LED diagnostic lights show all four on or when the screen is flickering the forth LED will flicker as well. I have tried plugging into an external display and get no image on the screen. I have tried replacing the logic board and the LCD. I'm thinking that it is an issue with the inverter but before I put out the money for a new inverter I'm hoping that there is a way to test the inverter or if anybody has a better idea of what the issue could be.