This is on a B8 Audi S-line but we all know the technology is the same the LCB super brain may help diagnose kit car lighting issues as well.

So, as per title. The light comes on but never really warms up and then finally goes out.

I don't think it is the ballast as the light does power up.

If I get a chance I'll change the bulbs left to right and see if it then works.

What do you lot think it is?

some of the modern lighting fails in 'mysterious' ways, and swapping side to side only confuses more. - the faults often don't seem to follow a logical path.
I think you need to test with NEW bulbs

Ballast

If it is HID then it is the ballast

I'd be tempted to check that all the connections are tight and clean - might be oxidisation causing resistance, or maybe a loose contact.

Thanks for the feedback. I doubt I'll have time to check today.

I never use it anyway so I am in no rush.

Gutted if it is the ballast, that is the more expensive part!

In my experience (limited and many years ago) if the ballast is faulty then nothing happens.

But, that's the power of LCB. Every day is a school day.