Still, the company is facing mounting pressure to figure out if a serious safety defect is causing the stalling problem, and to respond to customers who have experienced it. Some say they are hesitant to drive their Pacificas — or have stopped — until a fix or explanation can be found.
One owner, Adam Cohen, a lawyer in Leesburg, Va., said he and an advocacy group, the Center for Auto Safety, filed a petition with the National Highway Traffic Safety Administration on Monday calling on the agency to open a formal investigation of the stalling issue in Pacificas and to order a recall.
“I understand you have to find the source of the problem before you can fix it, but there’s still a safety issue here,” Mr. Cohen said in an interview. He purchased his 2017 Pacifica last year and said it had twice shut down while his wife was driving.
Fiat Chrysler says it is aware of the complaints and is looking into the matter, but has not found information pointing to a specific, widespread or consistent problem. The company “routinely monitors the performance of its vehicles using information from multiple data streams,” including formal complaints filed with regulators, Fiat Chrysler said in a statement.
The automaker “is unaware of any injuries or accidents associated with these complaints,” a spokesman, Eric Mayne, said by email. He added that Chrysler takes customer concerns seriously and that there was no indication that the function of airbags or seatbelt tension was compromised.
The Pacifica is designed so that its airbags remain operational even during a loss of power, he said.
That is a crucial safety matter. A faulty ignition switch that General Motors used in small cars caused the vehicles to switch off and rendered their airbags inoperable. The fault was linked to at least 124 deaths, most caused when crashes occurred and the airbags failed to deploy. G.M. eventually paid $900 million to settle a federal criminal investigation related to the faulty switches, set aside nearly $595 million to compensate victims of crashes, and recalled 2.6 million cars.
How Fiat Chrysler responds to complaints of possible defects is a sensitive matter. In 2015, the highway safety agency found that the automaker had failed to promptly notify vehicle owners of recalls and delayed initiating repairs of defective models in 23 recalls covering 11 million vehicles. The agency fined Fiat Chrysler $105 million.
In 2016, the actor Anton Yelchin was killed when his Jeep Grand Cherokee, made by Fiat Chrysler, rolled backward down his driveway and crushed him. The vehicle had been part of a recall related to its electronic gearshift, but had not been repaired when the accident occurred.
Both Mr. Cohen and Mr. Patel have left their Pacificas with their dealers and have declined to take the cars back until the stalling issue is cleared up. Mr. Cohen is driving a loaner vehicle. Mr. Patel, whose Pacifica had 308 miles on it when it shut down on the highway, is trying to get his dealer or Fiat Chrysler to buy back the minivan from him.
Mr. Cohen said he was particularly frustrated because he had been discussing the matter with Fiat Chrysler engineers and quality officials over the last several weeks via phone and email. On Oct. 22, he wrote to the company’s chief executive, Sergio Marchionne, to bring the issue to his attention.
“They want to put a data recorder on my Pacifica and have me take it back and drive it around,” Mr. Cohen said. “I told them they should drive it themselves and wait for it to happen. I don’t want my family to be their test dummies. And that’s where it stands. We’re at a deadlock.”
Kelly Katz, a mother of three small children in Yuma, Ariz., has also experienced the problem with her 2017 Pacifica. In May, she was creeping along in a parking lot when her minivan lost power. Like Mr. Cohen and Mr. Patel, she got a message on her instrument panel that said the vehicle had to be stopped, shifted into park and restarted.
A few weeks later, it happened while her husband was at the wheel. Her dealer drove the minivan for three weeks but never experienced the problem. The dealer did replace an electronic steering-control monitor, though.
She is still driving her Pacifica but it worries her. “I’m not O.K. with it,” she said. “I’ve got three kids in car seats. As a mother, you don’t want a minivan that can shut off while you’re driving.”