I had thought that the system (OEM) was just ballast and bulb, but when I look for spares on the Web I am seeing replacements for ballast, bulb, and a separate IGNITER.
My driver side HID fails a few seconds after switch-on. I found that the bulb had blown and replaced it but now it flickers brightly and then dies. Do I need a ballast or an igniter or am I completely wrong?

 

Swap the ballasts will tell you quickly. Remember they need to be off when you do this work.

 

I should have thought of that.