I just got a Phoenix DZBIS-112CII external flash for my Canon 550D. I have tried everything that I can think of to make it fire, without any luck. It came with four dead batteries in it which had caused some corrosion on the metal battery mount, but I doubt that is what is not causing it to fire. I will clean it soon anyway. The flash has four lights on its back: one for TTL, one for ON, one for flash (lightning bolt), and one for OK. So I turn the ON switch on and the ON light lights up. I think the lightning bolt should light up when the capacitors have filled up.. not too sure though (It shouldn't take very long, right?). It never lights up. The OK never lights up either. My two hunches are that there's an internal problem, or a problem with the hot shoe leads. Any diagnostics I could try?
Sounds like an internal problem with the flash indeed. Not the shoe.
The flash should be able to power up with fresh batteries. Once the capacitor is fully loaded it the button with the FLASH symbol should fire the flash. This is used also to distress the capacitors before shutting it down. So independent of camera settings.
Since you do get the ON light to light up it looks like it's not the battery contacts that are in fault. Anyway try to clean those contacts.
I'm afraid this is an internal defect. Be very careful not to shock yourself should you think of servicing it yourself.