CHICAGO  Jose Fernandez said he decided some time ago that on his salary as a restaurant worker, he was better off without his 1996 Toyota 4Runner. He hoped to make a nice bit of cash from its sale.

Before he could do that, though, someone beat him to extracting value: A thief sneaked under the sport utility vehicle with a battery-powered saw, slicing from the Toyota’s underbelly what may be one of the most expensive small parts of the auto world: the catalytic converter, an essential emissions-control device made with small amounts of metals more precious than gold. Who knew? Mr. Fernandez didn’t.

Inside the lobby of the New Windy City Mufflers and Brakes shop, Mr. Fernandez said he had heard a rumor that catalytic converters had suddenly become the rage on the black market here, but he did not believe it until his went missing on a well-lighted North Side street.

Theft of scrap metals like copper and aluminum has been common here and across the country for years, fueled by rising construction costs and the building boom in China. But now thieves have found an easy payday from the upper echelon of the periodic table. It seems there may not be an easier place to score some platinum than under the hood of a car.