The weakest link in most HID fixtures are the capacitors, Capacitors are easy to check, simple take a volt meter Touch 1 prong to a lead on the capacitor and the other lead to ground, then do the same for the other lead on the capacitor, the voltage levels should be pretty close, 1 lead is incoming voltage the other is out going voltage.  WARNING capacitors should always be considered hot, they hold voltage even when the circuit is not energized, to discharge a capacitor use needle nose pliers with a rubber handle, and touch the needle nose to both leads at the same time, sometimes they will make a pop,  Disclaimer: all testing and connections should be done by a certified electrician.

Item Description:
Capacitor Type:HID Capacitor
Lamp Type:Metal Halide
Lamp Watts:400, 1000
Capacitance (mfd):24.00
Voltage:480VAC
Construction:Oil-Filled
Case Shape:Oval Cylinder
Size:1.75'L x 1.90'W x 3.90'H
 

note: these don't work on an electronic ballast only on magnetic