*** Instruction before you buy the HID products***
When HID bulb goes out, there are 2 things to check: the Bulb and the
Ballast. To find out which part is bad, you can switch the working
side bulb into the side that doesn't work. If the bulb doesn't light
up on that side, then it means the ballast is bad. vise versa if the
bulb lights up, then it means the bulb had gone bad.
Rarely it is the ignitor or the bad wires, However these may also go bad.