DETROIT — If you live where salt is used to clear the roads of snow and ice, U.S. safety regulators have a message for you: Wash the underside of your car.

The message came Wednesday from the National Highway Traffic Safety Administration, which closed a five-year investigation into rusting pipes that carry brake fluid in about 5 million older Chevrolet, Cadillac and GMC pickups and SUVs, without seeking a recall.

Instead, the agency blamed the problem on rust caused by road salt and a lack of washing. It determined that it was not the result of a manufacturing or design defect.

The agency urged people in 20 cold-weather states and Washington, D.C., to get their car and truck undercarriages washed several times during and after the winter, and to get their brake lines inspected for rust and replace them if necessary. The warning underscores the importance of washing highly corrosive salt from beneath a car because over time, it can cause suspension parts, the frame, or other components to corrode and fail.

Most automatic car washes have jets that spray the undercarriage with water to hose off the salt. Some, like the Jax Kar Wash chain, based in Southfield, Mich., also can spray undercarriage rust inhibitor during winter months, said Shawn Connelly, a general manager.

NHTSA’s finding that the GM trucks weren’t defective came even though it received 3,645 complaints of brake pipe rust in the General Motors vehicles from the 1999 to 2007 model years, including 107 crash reports and 40 reports of injuries. Seventy-five percent of the complaints came from trucks in the first four model years covered by the investigation, 1999-2003, the agency said.

Investigators checked similar vehicles in Pennsylvania, surveyed owners in Ohio, and did random checks in other salt-belt states to determine that the same problem exists in just about every other vehicle from the same era because brake lines were all made of the same steel materials with aluminum coatings. The industry gradually switched to nylon or plastic-coated steel lines in the mid-2000s, NHTSA said.

The investigation started after NHTSA received a complaint from a Middletown, Ohio, man in March of 2010, who said the pipes that carry brake fluid on his 2003 Chevy Silverado rusted and leaked, causing a sudden reduction in braking power.

Yet it took until Wednesday for the agency to resolve the matter. The agency acknowledged that the probe took too long and said its Office of Defects Investigation is understaffed. Additional staffing and funding are in the Obama administration’s proposed budget, NHTSA said. The agency said that in recent years it also has concentrated on more serious recall issues that involved multiple crash deaths.

NHTSA’s car-wash advisory applies mainly to owners of vehicles from the 2007 model year or older in Connecticut, Delaware, Illinois, Indiana, Iowa, Maryland, Massachusetts, Maine, Michigan, Minnesota, Missouri, New Hampshire, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, Vermont, West Virginia, Wisconsin and the District of Columbia.