1. Visually check the bulb for blow out.
2. Check the bulb specification. 
3. If NG, replace the bulb with a new part.
CAUTION:
• Do not perform work with wet hands.
• When lit (lighting switch is ON), do not touch the harness, light internals, or metal parts of the light.
• When performing a lighting test, make sure that the headlight is mounted on the vehicle, and the power supply is connected to the connector on the vehicle’s side.
1. Check the bulb specification.
2. Install a SUBARU genuine HID bulb to check whether it lights properly.
3. If it does not light properly, replace with a new bulb.
4. Check the HID ballast.
Check the ballast in the following manner, and determine whether or not to reuse.
(1) Perform cold starts (light the headlight after turning off for 10 minutes or more), and hot starts (light the headlight for 15 minutes or more, then turn off for 1 minute and relight) several times, to make sure that the headlight lights properly.
(2) Monitor the lighting condition right after a cold start up to a stable state (approx. 5 min.), to see if there are any unstable conditions such as flickering.
(3) Attach two bulbs with the equal amounts of operating hours to the right and left headlights, and light the headlights for approximately 30 minutes. Check for whether there is a difference in brightness on the right and left sides.
5. If there is a malfunction, replace with a new ballast.