Arlington, Va. ? At least a couple of times a week, mechanic Ernie Pride tells customers at his independent repair shop he can’t fix their cars because he doesn’t know what’s wrong with them. Go to the dealer, he advises.

He has the experience and knowledge to service vehicles but lacks the closely guarded information needed to diagnose problems with today’s high-tech cars.

Automakers refuse to make much of it available to independent shops that compete with higher-priced dealerships. The practice is raising hackles in Congress and a vigorous defense by the industry.

Figuring out what’s wrong with an automobile is no longer as simple as poking around under the hood and examining parts. Computers control many modern vehicle systems, including the engine, the air bags and the antilock brakes. Mechanics now diagnose problems by connecting a handheld computer to the vehicle.

The computer gives the mechanic a code of numbers or letters that designate the source of a problem. Without the reference material to interpret the code, a mechanic can’t fix the car.

“We just say, ‘We’re sorry. You’ve got one option go to the dealer,”‘ said Pride, manager of The Car Store outside Washington.

All repair shops must get some emission system codes because of the Clean Air Act.

Some members of Congress worry that higher-priced dealer repair shops are using the codes to corner the repair market. Lawmakers have introduced legislation to require manufacturers to share diagnostic codes with car owners and independent repair shops.

Also, the Environmental Protection Agency is developing a plan to require that automakers publish online all the codes related to emission repairs.

Cars built since the 1996 model year must have computer-controlled emission systems to meet clean air laws.

“Most vehicles out of warranty are serviced by independent repair shops,” EPA spokesman David Ryan. “And the sooner these shops catch emission problems, the better it is for the environment.”

A membership survey by the Automotive Service Assn., which represents 15,000 independent repair shop owners, found that 10 percent of cars could not be repaired because codes are not available. The number is expected to grow as newer cars replace pre-1996 models.

The Alliance of Automobile Manufacturers says that requiring the codes’ disclosure would make proprietary information available to competitors and subject to copying.

Automobile dealers made a record $80 billion on service and parts in 2001, an 8.5 percent increase over 2000. Dealer labor rates tend to run from $10 to $20 per hour higher than independent shops, according to AAA.

Dealers contend it is appropriate that they have access to sensitive information while independent garages do not.

“Dealerships have a franchise relationship with the manufacturer, and the manufacturer can terminate that relationship,” said Doug Greenhaus, director of environment, health and safety for the National Auto Dealers Assn. “They are under contract to keep that information confidential, but there is no relationship like that with the vehicle manufacturer and the aftermarket.”