Allston auto mechanic Jerome Brasseur hates turning away business, but he doesn’t always have a choice.

Brasseur cannot make simple fixes on some cars because he doesn’t have the diagnostic codes he needs. He has to send owners of certain BMW and Mercedes models to manufacturers’ dealerships for something as basic as a transmission fluid change because automakers consider such information proprietary and are reluctant to divulge it to independent repair companies.

“They make it [so] that only the dealership can change the transmission fluid,’’ said Brasseur, whose JCB Auto shop is not affiliated with an automaker. “You kind of know after a while what jobs to walk away from, where you say to the customer, ‘Yeah, sorry, I’m not going to be able to help you.’ ’’

That presents consumers with a dilemma: They can take their car to a shop that might not be able to make a fix, or bring it to a dealership and risk spending more money.

Repairing autos is an increasingly complex and computerized job, requiring specialized diagnostic tools that help a mechanic obtain repair information and codes from a vehicle’s on-board computer system. Shutting off the “check engine’’ light on some cars, for example, requires a mechanic to punch in a special six-digit code created by the manufacturer.

A coalition of Massachusetts businesses is lobbying for a bill called the Right to Repair Act that would guarantee independent mechanics and shop owners access to the diagnostic data dealerships get. The bill is under review by the Legislature’s Joint Committee on Consumer Protection and Professional Licensure, and more than 40 legislators have expressed support for it.

Auto manufacturers are lobbying against the measure, saying it could force them to reveal trade secrets, such as a vehicle’s electronic control module. The module is essentially a car’s brain, controlling everything from air bags to the diagnostic system.

Plummeting car sales have heightened dealership opposition to the bill. Ami Gadhia, policy analyst for Consumers Union, publisher of Consumer Reports magazine, said that’s because dealerships now rely on revenue from repairs, not sales, to turn a profit. Gadhia said Consumers Union supports the bill in principle because it promotes competition between dealerships and independent shops.

“The more competition, the better the choice for consumers and, obviously, the better the pricing,’’ Gadhia said.

Charles Territo, spokesman for the Alliance of Automobile Manufacturers, a national group of 11 companies, said after-market parts manufacturers, such asAutoZone or Napa, want information about electronic control modules and other parts so they can reverse-engineer them and, ultimately, manufacture and sell the same gear at a discount.

“That is highly guarded trade proprietary information and manufacturers spend billions and billions of dollars researching and developing it to make their products unique,’’ Territo said. “They want to get all that for free.’