Offside bulb turns off after 20 seconds or so. Is it safe to assume it's HID / Ballast related?

I'd swap the bulbs over to check, but I don't have any of my tools at the moment.

If it is the ballast, what are my options for getting it working again? Guessing Subaru part will be about £999,999.....

 

Probably water in the ballast.
Remove from headlamp and dry out and give then it a try.

If not plenty of new and S/H Matsu****a-Panasonic replacements about.
You don't say what year/revision we are talking about so
Revision C (pre facelift) part number 84965KG000


Revisions D,E and F (facelift) part number 84965SA011

 

SG9 I think is facelift (smoother bumper / no foglights)? 2007. Cheers for the suggestion. It has been bloody wet recently.

 

SG9 equates to 2.5L engine which all FSTI's are. (S being Forester:G being 2nd Generation; 9 being 2.5 L or 5 being 2.0L)
2007 will be a revision E but most likely an F (4th character of the Applied model code) as model years overlap.

 

Great info. Much appreciated.

Swapped the bulb over to the other side and it worked, so definitely ballast (unless there is a separate fuse on both sides or something). Found a new one here: https://xenons4u.co.uk/matsu****a-pa...-unit-ecu.html
Is there a secret to getting at them? I didn't have enough time before sunset to investigate further, but the lead seemed to go somewhere I'm not sure how to reach. Probably go in through the boot right? (joke)

 

The ballast is attached to the underside of the headlamp.



To remove the headlight it is necessary to remove the front bumper first. (see pdf)



Having removed the headlight the ballast can be removed (section2 of headlight pdf )

 

Wow @Don Clark That's some awesome info. Thank you so much!!