Doug Decker's friends in high school knew how to cheat state emissions tests in their modified Mustangs and Firebirds: Just before the inspection, they'd adjust the carburetor to fill the engine with more air and less gasoline. The car would generate less power, but the fuel would burn completely, and the exhaust would come out clean.

"It was the way you played the game," said Decker, who is now in charge of motor-vehicle pollution for the Colorado Department of Public Health and Environment.

"It's still the same game," he added. This week, Volkswagen admitted that it manipulated some 482,000 diesel engines to bamboozle inspectors. Some automotive experts see vulnerability in government emissions-testing programs, despite their cost and inconvenience for drivers.

Regulators say that Volkswagen's so-called "defeater device" used sensors for pressure, temperature, engine speed and more to register when the car was being put through the minutely prescribed federal testing procedure. If the onboard computer detected a test underway, it would reduce the car's emissions. On the road, the car would run normally, likely with improved gas mileage and performance but emitting nitrogen oxide at as much as 40 times the federal limit, according to regulators.

"Our company was dishonest with the EPA and the California Air Resources Board and with all of you," VW's U.S. chief, Michael Horn, said Monday.

Testing allows the Environmental Protection Agency to enforce its fuel economy standards, but since the protocols are published in advance in detail, manufacturers are able to design cars that perform better on the tests than on the road, Bishop argued. Outright manipulation isn't always necessary.

Tests mandated by state and local governments didn't detect anything amiss, because few states test diesel engines in light-duty vehicles. What's more, diesel inspections generally don't test for nitrogen monoxide, the primary oxide of nitrogen produced by diesel engines, which is colorless and odorless.

For drivers with gasoline engines, these simpler tests are far easier to cheat than the federal inspection, a comprehensive shakedown that can take a few hours.

Gearheads might once have tinkered with an engine's timing using a screwdriver, but these days, they can buy aftermarket computer chips for better acceleration or towing capacity.

These chips sometimes mean more pollutants, too. An extreme example is the practice of "rolling coal" -- disabling the pollution-control program in a diesel truck's computer so that the vehicle spouts a plume of thick black exhaust -- which has no other purpose other than mocking environmentalists on the road.

Drivers can unplug these modifications before taking the car in for mandated testing.

Inspection "costs lots of money," said Gary Bishop, a research engineer at the University of Denver. "It does absolutely nothing to clean up the air."

Bishop's laboratory has developed a roadside sensor, which he and his colleagues have been using for more than a decade to see how cars actually do on the street in several major cities. The results allow Bishop to guess as to what modifications drivers have made to their cars. A Honda Civic with an unusual profile in the emissions data probably has a street-racing chip, for example.

And some vehicles are obviously malfunctioning and should be taken off the road. "They're putting out so much unburned gasoline that you could actually run another car off of their tailpipe," Bishop said.

One of the cities where Bishop has worked is Tulsa, Okla., where emissions tests have never been required. The group has found that emissions from the cars in Tulsa are no worse than emissions in other cities where standards are enforced.

Authorities are now using the sensors in and around Denver and in a few other states as a supplement to conventional testing. The state sets up the sensors at highway on-ramps and elsewhere along the road. Drivers don't stop. They just roll between two rows of cones while a camera records the car's license plate and the equipment registers the emissions from the tailpipe, and go on their way. If a car produces at least two passing grades, the driver is spared the trip to the inspection station.

The sensors clear about a third of the vehicles that would be due for inspection in any given month, said Decker of the Department of Public Health and Environment.

"That's a tremendous convenience," he said. "Remote sensing has been a wonderful thing for the people of Colorado."

If a car's fumes are suspicious, though, there is little the state can do beyond ask the driver to bring it in for inspection. Remote sensing can't solve all the problems of testing for emissions.

All the same, the technology is attracting policymakers' attention.

Nevada will be putting the sensors along its roads in the coming years. State Assemblyman Jim Wheeler called remote sensing "the only true test right now."

Before he became the Republican whip, Wheeler was the head of a company that manufactured aftermarket car parts. He predicted that manipulation on emissions tests will continue. "As technology continues to advance, we're going to see more and more of this, especially in the aftermarket-parts arena," Wheeler said. "By, I think, the end of the decade, you're going to see a lot of states going to drive-by testing."