They're attached to a black keypad on the dashboard and start flashing on the first day a car payment is late. On the fourth day, after two more days of warning lights, the car won't start.

"I would not undertake buy-here/pay-here without this system," said Ray Williamson, president of North Texas Motorcars, which sells about 50 vehicles a month and installs boxes in each of them. "There's just too much risk."

The box from a Temecula company – called a starter interrupt unit – is used mostly at used-car dealerships that provide financing to customers with bad credit. But other segments of the auto industry may adopt it, particularly if consumers' credit ratings continue to decline.

"I can see this coming," said Michael R. Linn, chief executive of the National Independent Automobile Dealers Association, which is based in Texas and represents 19,000 used-car dealers nationwide.

"The technology is there. Look at something like General Motors' OnStar. It can already open doors and notify emergency authorities, and all of that. It could certainly shut a car down."

The On Time unit Williamson uses is marketed by Temecula's Payment Protection Systems and is one of three or four such systems available. Mike Simon, president and chief executive, said On Time sales have increased 40 percent since 1999.

The company has begun selling On Time units equipped with GPS that some mainstream used-car dealers are installing on $20,000-plus luxury cars.

"Some people have the income and assets to buy a Mercedes-Benz but still have credit problems," Simon said.

"This takes a lot of the risk out of it, just like on the lower end."

He believes the devices could be used in the new cars as well.

"In the auto industry, there are prime and subprime buyers in every segment," Simon said. "Even though you qualify for a new Ford or an SUV, you might still be subprime because of your credit history. You can put this in a vehicle and probably get the buyer a lower interest rate."

Disabler devices have been available for three years and are used at about 15 percent of the tote-the-note used-car lots in the United States, officials estimate.

The customers typically have damaged or no credit and pay interest rates up to 26 percent. The vehicles often have 70,000 or more miles on them and cost $7,000 to $10,000 – or roughly $70 to $100 a week.

Andrea Gooden, a nurse at a geriatric-care facility in Dallas, has bought four cars with the devices on them and says she accepts them. Gooden drives a 2002 Honda Accord and is helping a daughter at the University of Houston with her car payment.

"The deal was, I make the first payment of the month and my daughter makes the second," said Gooden, 46, of Mesquite, Texas. "It warned her when she didn't make the payment on time, and when it got to four days, the car quit running. She learned not to do that again."

Gooden said the system helped reduce the interest rate on her car to 13.9 percent.

Note lots have long been the auto industry's roughest segment. But without them, industry officials say, many working-class people would have to rely on public transportation to get to their jobs. About 35 percent of the nation's 54,000 used-car dealers operate buy-here/pay-here lots and contend with default rates as high as 40 percent.

Systems such as On Time permit dealers to "drill down" even deeper with severely credit-damaged customers and still have some assurance that their only collateral on the high-risk loans – the vehicles – can be retrieved if the buyers default.

"Nearly every Saturday, we talk with someone who has been to seven or eight different buy-here/pay-here lots and were turned down at all of them," Williamson said.

"More people in this country have bad credit than have good credit," said James Ziegler, a retail auto consultant in Duluth, Ga. "If somebody didn't take a chance on these people, they wouldn't be able to work."

Simon of On Time said the systems dramatically reduce late payments.

"We have taken a 70 percent delinquency rate in the buy-here/pay-here business and transformed it to a 96 percent on-time rate," he said. "Now some mainstream finance companies are even saying, 'I'll finance these cars.'"