Equal chance of the bulb or the controller (ballast) - you can swap the bulbs side to side to see where the fail goes. You can't usually see any issue with the bulbs.
Any HID bulb will work with any controller *provided* they are the same wattage rating - usually either 35W or 50W. The wrong bulb can kill the controller. You may find that with the same bulbs and different controllers that the colours don't match exactly.
No problem swapping back to standard bulbs; just remove the existing controller, bulb, and wires, and connect the earth wire back to the tag on the holder in the headlight, and the live feed to the bulb base. Don't touch the bulb with your fingers; use gloves if you need to.
Edit: the bulbs and controllers are all over eBay. It's probably cheapest to get a complete kit and keep the old bulb/kit as spares. Bulbs dim over time.
Edited by barnacle (29/07/2017 21:47)