On Nov, 25, 1977, a man drove a late model sedan into a Georgetown service station and complained to a mechanic that the car was running roughly.

Later, the man got a bill for $65 to cover carburetor adjustments and cleaning, spark plugs, general tuning and labor. The car could have been fixed for $1, police said.

Yesterday, the mechanic pleaded guilty in D.C. Superior Court to one count of attempted false pretenses in connection with his services and the car.

In fact, the sedan is owned by the consumer fraud unit of the D.C. police and its driver is and undercover police officer. Its ills, if there are any, are meticulously cared for by a mechanic with 35 years experience, who documents his every move.

The unit's mission is to combat auto repair fraud.

Since last summer, the sedan, and several other cars owned by the fraud unit, have been driven to about a dozen Washington service stations. The consumer unit has received complaints about, according to Sgt. Walter J. Franek, the unit supervisor.

Each time the police mechanic certifies the car to be in perfect working order, and deliberately arranges a minor and obvious defect under the hood. The car is then taken to an auto repair shop for repairs.

The investigation, Sgt. Franek said, is both time-consuming and expensive as law enforcement officials try to draw the line between simple inefficiency or a legitimate mistake by a mechanic and the actual commission of a crime.

The Georgetown mechanic, William Derrick Mass, 24, of 1315 N. Ode St., Arlington, faces a year in prison, a $1,000 fine or both for the offense, which is a misdemeanor. He is scheduled to be sentenced May 10, according to the U.S. attorney's office.

Mass is employed at Lad Mills Exxon, a service station located at 1545 Wisconsin Ave. NW. Assistant U.S. Attorney William S. Block emphasized yesterday that "there is no evidence to suggest that (Mass' conduct) was a practice of the station."

"He got a bad deal," said Ted Olmstead, the general manager of the service station, of Mass, whom Olmstead described as a "damn good mechanic."

Mass, the serive station mechanic, said that he had to disassemble the carburetor as well as clean and reset parts of the car in order to get it running smoothly, law enforcement officials said.

Subsequent investigation revealed that the carburetor had not been taken apart, police said. In addition, while Mass had in fact corrected the minor defect created by the police mechanic, he did not inform the undercover policeman "consumer" that he had discovered that problem.

Last year, the Law Enforcement Assistance Administration gave the consumer fraud unit a $26,200 grant to purchase "undercover" cars and train officers to pursue auto repair fraud, according to Sgt. Franek. The project is not funded by the Police Department, he said.

Franek said about 200 complaints about auto repair problems were brought to the police fraud unit last year - second only to the number of complaints received about the home repair business.

Joan Claybrook, chief of the National Highway Traffic Cafety Administration, testified at Senate hearings this week that an estimated $20 million was wasted by consumers last year on "inadequate, incompetent, unnecessary and fraudulent auto repairs and maintenace . . ."

The hearings, completed yesterday, were conducted by the consumer subcommittee of the Senate Commerce Committee in connection withproposed legislation that deals, in part, with auto repairs.