Replace Leaky HID Headlight Assembly Early

If your Prius HID headlight assembly is fogging with water after a rain or car wash, my advice is to replace it immediately.

One of the Taiwanese headlight manufacturers (DEPO) has recently begun making replacement HID assemblies at a fair price of ~$200 per side.

The reason to replace early is the leaked water accumulates in the bottom of headlight assembly where the HID electronic ballast resides. When water hits the ballast, it dies. Replacement ballasts cost another $150 per side.

There are several YouTube videos that show how to remove the headlight assembly.

The reason that DEPO charges so much more (2x) for the HID version of the headlight than for the halogen one is that the HID version comes with a ton of hardware necessary to tilt the lamp when you press the high beam button. The tilting mechanism is quite complex. DEPO chose to include the tilting hardware with the new headlight rather than require you to transfer the old hardware to the new headlight.

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This headlight replacement effort has given me new insight into the politics behind the 15 year old customer relations fiasco Toyota created with Prius' HID headlights. I have engineering background in the HID lighting industry, so the matter of HID lamps that won't ignite early in their operating life is familiar to me.

At the time the fiasco began Toyota identified the HID lamp (aka "bulb") as the failed item. But the truth was that the ballast's design engineer didn't endow it with sufficient ignition voltage to handle the demands of aging lamps. In other words, the electronic ballast had a design flaw.

I was curious at the time why Toyota didn't simply recall HID-equipped Priuses for ballast replacement. (BTW, I've never had HID lamp problems in my 2009.) The reason is quite apparent now that I have replaced my own HID headlight assemblies. The front bumper cover needs to come off to remove the headlights, and then you discover that the ballasts are embedded into the headlight assemblies and cannot be easily removed from them.

Worse yet, you can't remove the ballast from the headlight assembly without severing its high voltage silicon wire harness. The plug on the end of the high voltage harness is too large to pull it through the cavity in the headlight. When the headlight was constructed the harness was threaded in a way that cannot be reversed after the headlight is sealed.

Bottom line: Gen II Prius' entire HID headlight assembly was flawed from the start. Toyota believed the lamp and ballast would last through the warranty period and beyond and made no provisions for low cost replacement of the ballast. And I don't think they realized that the domed cover over the back of the HID lamp would seize up and not twist off after the headlight assembly aged.