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Discussion Starter #1
Drivers side headlight went out last week. Tried to replace using OSRAM 66340 and not getting it to work. Removed bulb is OSRAM 66340HBI. Both are D3S. I thought they were incompatible.

Is this a bulb issue or should I be troubleshooting other problems?
 

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Those two bulbs are interchangeable. Could have an issue with ballast. Try swapping the driver and passenger bulbs to see if the known good bulb from the passenger side works on the driver side.
 

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Discussion Starter #4
Thanks 89lxvrt! Did the the test and it’s not the bulb. New OEM ballast seems hard to find under $100. Any recommendations on aftermarket ballast and install? Or is it better to just suck it up and buy the OEM part?
 

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Thanks 89lxvrt! Did the the test and it’s not the bulb. New OEM ballast seems hard to find under $100. Any recommendations on aftermarket ballast and install? Or is it better to just suck it up and buy the OEM part?
You could also swap ballast side to side to prove that is the issue. If you want to be sure.

I bought used ballast when I changed to D3S in my 2013 ST3 headlights. The Osram ballast is not car manufacturer specific, mine actually came from a Tesla. Just search eBay for whatever numbers you can find on the ballast and match up the picture to what you have. I paid less than $100 for two bulbs, ballasts, and wires. You should be able to find a good deal on a single used, tested, and guaranteed.
 
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