Genie in a Bottle? Testing a Headlight-Lens Defogging Solution As a car ages, its plastic headlight lens covers tend to fog and yellow, which can reduce the amount of light that hits the road. Though it's not as routine as an oil change or a tire rotation, replacing fogged lenses is no less critical maintenance for car owners. The high cost of replacement, however, has begotten a cottage industry of headlight restoration kits, sold at all major automotive supply stores. Their manufacturers say the products can remove the discoloration and return a headlight to like-new condition. That is, if you can get them to work. A few years ago, I used an off-the-shelf product to clean the yellowed lens covers on my family's 1996 Chrysler Town & Country. It had little effect, and I had the entire headlight assemblies replaced for $465. The lens covers on my recently acquired 2001 BMW 325xi were fogged. Balking at the $80 lens-replacement costs I had found in my research, I tried the Quixx Headlight Restoration Kit and Lens Sealer, which came on the market in spring 2011. Quixx Care System, a company based in Munich, claims its kit restores lenses to new. I was bullish from the get-go; even the packaging depicted the unmistakable headlights of a BMW, in "before" and crystal-clear "after" condition. Though some lens cleaners come in a single bottle of liquid solution, the Quixx kit, which costs $19.99, came with a tube of plastic polish; lens sealer; a sanding block with three different ultrafine sanding papers at 2,000, 3,000 and 5,000 grit; two polishing cloths; and two special-application cloths. With the driver's side headlight wiped clear of any debris, I applied the plastic polish with one of the appropriate cloths. I then wet-sanded the lens, starting with the 2,000-grit paper and working my way through the finer sheets. I finished the process by applying the sealer and, when dry, repolishing the lens. The improvements were noticeable, with greater clarity and a touch less yellowing. Even so, I wouldn't mistake the lens for that of a 2012 3 Series. Using a buffer may have brought better results, but the Quixx instructions didn't call for one. Meanwhile, impressed but not wowed by this $20 remedy, I did what everyone does when they have a car problem they can't solve themselves: I went to my repair shop. Joe Parente, co-owner of Soundview Service Center in Old Greenwich, Conn., said restoration products rarely work. The only way to return headlights to like-new condition is to buy new ones, he said. Some customers who insist on trying a yellowing-removal treatment instead of a headlight replacement may just end up paying twice for the same result. "I'll tell them we can give it a shot, but in most cases we have to replace the headlight," he said. Phil Sidoti, a technician at the shop, disagreed with his boss. Mr. Sidoti swore by the 3M Headlight Lens Restoration System, which he said was the best way to clear yellowing headlights without replacement. But both technicians agreed that when the discoloration was on the inside of the headlight, there was no choice but to go the replacement route. Try though I did to avoid it, that seemed to be exactly where I'd be headed.