DEATH VALLEY NATIONAL PARK, Calif. — It wasn't quite 11 a.m., but the thermometer at Stovepipe Wells already read 117 degrees.

Bob Ditner smiled. The late summer heat was just what the Toyota Motor Corp. engineer needed to test the new front brakes on a 2006 Lexus sedan.

For two hours, he ran the Lexus on a winding, two-lane road that quickly dropped from 5,000 feet on the western rim to sea level on the Valley floor. He hit the brakes more than 100 times — instruments recorded every detail — as the temperature of the brake pads soared to more than 800 degrees. At that temperature, brake fluid would boil if the system wasn't properly insulated, and air bubbles would form in the brake lines, making the brakes inoperable. Under those circumstances, when a driver hits the brake pedal, it "will go right to the floor and you'll keep on going," Ditner said.

Driving at 60 mph at the bottom of the slope, he slammed on the brakes. The Lexus stopped straight, true and quickly. Hot brakes may "seem like a little thing, but many drivers use their brakes constantly going downhill, and that's what we want to protect against," Ditner said.

Despite the use of increasingly realistic computer simulations, his crew was in Death Valley doing what automakers still have to do to new cars: Give them road tests under the toughest conditions imaginable. In the auto trade, they call it extreme testing.

Each year, automakers introduce about a dozen all-new passenger vehicles, while a hundred or so other models are delivered with major changes to transmissions, air conditioning, lights, suspensions, even door seals. The typical car or truck has about 3,000 parts, and automakers test new pieces individually, often with computer simulations or in laboratories. But ultimately these pieces must be bolted onto a car and put on the road to find out how they all work — or don't — by exposing them to extreme bumps, bangs, heat, cold, dirt and dampness.

It's a crucial step to keep product quality high because reliability surveys by J.D. Power & Associates, Consumer Reports and others can heavily influence car sales.

"We have a motto, 'Genchi gimbutsu,' that translates as 'Go and see,' " said Chuck Gulash, director of testing at Toyota's U.S. technical center in Ann Arbor, Mich. "We go and see for ourselves how something will function in adverse conditions."

Toyota has more than 1,000 engineers and technicians focusing on extreme testing, and Ford Motor Co. has about 1,500 test engineers. Other automakers have similar staffs. Their extreme road tests have pinpointed air filters that let dust into passenger cabins, brake parts that overheat and crumble, and wiring harnesses that fray and crack in extreme cold.

To run these tests, carmakers set up hot-weather test tracks in Arizona and cold-weather facilities in Northern Canada. They also road-test cars on grueling climbs up Colorado's Pike's Peak, in Arizona's choking red dust and in Mexico City's helter-skelter traffic. Overseas, winter road tests are done in Scandinavia and the Alps, and summer spins are in Spain or in the Algerian and Australian deserts.

Death Valley is a favored summer test spot because its broiling sun bakes plastic parts, toasts paint and stresses radiators and air conditioners.

"Everyone uses it," said Ditner, who pointed out rival test crews from Nissan, Mercedes-Benz and Aston Martin during his Lexus braking run.

Riding with Ditner was a senior Toyota brake systems engineer from Japan while two other engineers drove behind in a rented minivan, watching the Lexus for signs of distress such as smoke from burning brakes pouring from the front wheels.

Toyota requires that prototypes of new cars be driven for 100,000 miles. For the U.S. market, that is usually done on the company's 10-mile test track outside Phoenix. Alternating shifts of drivers run the vehicles 20 hours a day, five days a week, for as long as 10 months.

These days, automakers have compiled so much data on so many components that most new parts or systems can be tested accurately with a computer or automated simulation.

Ford has an indoor facility in Michigan that duplicates many climate, elevation and road conditions. Inside, monsoons are fed from overhead rainmaking nozzles and giant blowers create winds that drive blizzards made in snow machines. The test building can be pressurized to simulate varying elevations.