Borrowers are typically provided with codes that are supposed to restart the vehicle for 24 hours in case of an emergency. But some drivers say the codes fail. Others say they are given only one code a month, even though their cars are shut down more often.
Some drivers take matters into their own hands. Homemade videos on the Internet teach borrowers how to disable their devices, and Spireon has started selling lenders a fake GPS device called the Decoy, which is meant to trick borrowers into thinking they have removed the actual tracking system, which is installed along with the Decoy.
Oscar Fabela Jr., who said his 2007 Dodge Magnum was routinely shut down even when he was current on his $362 monthly car payment, discovered a way to circumvent the system.
That trick came in handy when he returned from seeing a movie with a date, only to find his car would not start and the payment reminder was screaming like a burglar alarm.
"It sounded like I was breaking into my own car," said Mr. Fabela, 26, who works at a phone company in San Antonio.
While his date turned the ignition switch, Mr. Fabela used a screwdriver to rig the starter, allowing him to bypass the starter interruption device.
Mr. Fabela's car eventually started, but it was their only date.
"It didn't end well," he said.
Across the country, state and federal authorities are grappling with how to regulate the new technology.
Consumer lawyers, including dozens whose clients' cars have been shut down, argue that the devices amount to "electronic repossession" and their use should be governed by state laws, which outline how much time borrowers have before their cars can be seized.
State laws governing repossession typically prevent lenders from seizing cars until the borrowers are in default, which often means that they have not made their payments for at least 30 days.
The devices, lawyers for borrowers argue, violate those laws because they may effectively repossess the car only days after a missed payment. Payment records show that Ms. Bolender, the Las Vegas mother with the sick daughter, was not in default in any of the four instances her ignition was disabled this year.
PassTime and the other manufacturers say they ensure that their devices comply with state laws. C.A.G. declined to comment on Ms. Bolender's experiences.
State regulators are also examining whether a defective device could endanger the borrowers or other drivers on the road, according to people with knowledge of the matter who spoke on the condition of anonymity.
Last year, Nevada's Legislature heard testimony from T. Candice Smith, 31, who said she thought she was going to die when her car suddenly shut down, sending her careening across a three-lane Las Vegas highway.
"It was horrifying," she recalled.
Ms. Smith said that her lender, C.A.G. Acceptance, had remotely activated her ignition interruption device.
"It's a safety hazard for the driver and for all others on the road," said her lawyer, Sophia A. Medina, with the Legal Aid Center of Southern Nevada.
Mr. Pena of C.A.G. Acceptance said, "It is impossible to cause a vehicle to shut off while it is operating," He added, "We take extra precautions to try and work with and be professional with our customers." While PassTime, the device's maker, declined to comment on Ms. Smith's case, the company emphasized that its products were designed to prevent a car from starting, not to shut it down while it was in operation.
"PassTime has no recognition of our devices shutting off a customer while driving," Ms. Kirkendall of PassTime said.
In her testimony, Ms. Smith, who reached a confidential settlement with C.A.G., said the device made her feel helpless.
"I felt like even though I made my payments and was never late under my contract, these people could do whatever they wanted," she testified, "and there was nothing I could do to stop them."
Articles in this series are examining the boom in subprime auto loans.
Read MoreIn a Subprime Bubble for Used Cars, Borrowers Pay Sky-High Rates
—By Michael Corkery and Jessica Silver-Greenberg, The New York Times