Could just need a new pair of bulbs:
How long to D2S bulbs last?
Unlike halogen bulbs, which typically die when the filament snaps, HID bulbs have very few methods of mechanical failure. This results in bulbs can last an excessively long time (thousands and thousands of hours), but as time goes on the output of the bulb decreases. According to some spec sheets, at around 1500 hrs of use, an HID bulb is only emitting around 75% of its brightness. So assuming linear lifespan (which is likely not a valid assumption, but for clarities sake we will assume it is), at around 3000 hrs, the bulb is at 50% brightness. This means, a bulb may still be working long after its luminous output has fallen below that of a halogen bulb.