Today’s cars contain sophisticated engine control software that determines how a vehicle’s engine operates, including fuel efficiency and exhaust emissions. In Volkswagen’s case, software could detect when a car was being tested for emissions and activate its pollution controls, then deactivate them when the car was being driven normally.

Carmakers and companies that supply emissions-testing equipment have long been on the lookout for tinkerers. Mr. Norris, the Indiana car customizer, said that auto companies began using special techniques about a decade ago to detect if a car’s factory-installed software had been altered. Altered software, he said, could give the carmakers a reason to void the warranty.

One emissions-testing company, Opus Systems, uses a patented method to identify cars equipped with devices built to fool state inspection equipment. “Our method is meant to catch fraud by individual car owners,” said Lothar Geilen, the chief executive of Opus, which supplies emissions-testing equipment to 22 states.

The latest avenue for engine tinkering was opened last month when the Library of Congress, which oversees the United States Copyright Office, approved several new exemptions to the Digital Millennium Copyright Act, a far-reaching 1998 statute that was meant to bring copyright law into the digital era. Every three years, federal officials review potential exemptions to the rule.

As part of the latest three-year review, the Electronic Frontier Foundation, an advocacy group based in San Francisco, filed petitions seeking changes to parts of the laws that prohibited consumers from unlocking software “access controls” on certain products. Two of the petitions dealt directly with vehicle software, including software used to control a car’s engine and emissions-control units.

Kit Walsh, a lawyer with the Electronic Frontier Foundation, said the group believed that researchers and car owners needed access to vehicle software not only to make repairs or to adjust a car’s performance, but also to improve security. To demonstrate security vulnerabilities in automotive software, two security experts, Charlie Miller and Chris Valasek, recently performed an experiment in which they remotely hacked into a Jeep Cherokee and took control from the driver.

Mr. Walsh said many companies that make auto components supported the changes to the law. One such company, Derive Systems of Sanford, Fla., reprograms engine computers in ways that it says improve fuel efficiency while reducing emissions.