Flickering: Flickering on both sides of the vehicle can be caused by insufficient available power or incompatibility with the vehicle's computer system.
Please determine if the flickering occurs with the engine running and with the engine off. If the problem occurs only with the engine running, it's likely that you need error code eliminators to make the HIDs acceptable to the vehicle's computer system (Dodge Rams and later model BMWs are exceptions as the Type 3 Error Code Eliminators will not correct the problem).
If the flickering occurs with the engine running and with the engine off, there may be a general powering issue, which may involve DRLs (see below) or be a general powering issue that may be resolved by installing an HID accessory harness so that the HIDs are powered from the battery rather than through the stock wiring.
If your car has Daytime Running Lights (DRLs), the reduced operating voltage of the DRLs is likely responsible for the flickering; the problem may be corrected by pulling the fuse for the DRLs or otherwise disabling the DRLs (this may not be legal in your jurisdiction so please check the local laws). An accessory HID harness may solve the flickering problem due to DRLs but this will convert your DRLs to "full on" status, or check local forums or google on how to turn off your DRL for you specific vehicle.
Neither light turns on: This is almost always due to reversed polarity. This can be corrected by reversing (flipping) the connector on the input side of the ballast. Another possible explanation is a blown fuse. Or, if a harness is being used, the relay or in-line fuse has likely failed. (under information section we have more detailed guide)
One side doesn’t turn on reliably or at all: This is likely caused by a defective bulb, ballast or an issue with the stock wiring (available power). To identify the responsible component, please swap bulbs or ballasts from one side of the vehicle to the other; if the problem follows one component then that is the part responsible for the failure. If all of the components prove to be functional on one side of the vehicle then you likely have a powering problem on the side that is failing; this will likely be corrected by installing an accessory HID harness.
Intermittent firing for both sides: If it takes multiple on-off cycles to get the HIDs to turn on, you may have a powering issue, which is often resolved by the installation of an accessory HID harness. Also, sometimes increasing the headlight fuse by 5 amps solves the problem.
If you have an HID Harness (whether it's an accessory or H/L harness) installed and are still getting intermittent firing, double check your power and ground cable connections.