But Toyota and analysts said they did not expect the problem to slow sales of the Prius or hybrid vehicles in general, which have jumped as drivers seek better fuel economy in the face of soaring gasoline prices.
The software problem first came to light in May when the National Highway Traffic Safety Administration said it was investigating complaints from Prius owners about engine stalls and shutdowns.
"We have a total of 428 complaints, but no crashes or injuries," agency spokesman Rae Tyson said.
"We're satisfied that their remedy is going to take care of the problem," Tyson said.
As a result, "we intend to close our investigation" and Toyota is "to be commended" for the voluntary recall, he said.
The system uses complex electronics and computer programming to make the car's separate gas engine and electric motor work together smoothly.
Toyota will notify Prius owners by mail that they can take the car to a dealership for free repairs, said Allison Takahashi, a spokeswoman at Toyota's Torrance-based U.S. operation.
If there is a stall, "you still have your electric motor and your brakes and your steering, so the driver can pull over," away from traffic, Takahashi said.
Honda Motor Co. and Ford Motor Co. are among the firms that also have rolled out hybrids as demand for the vehicles has increased.
Toyota does not expect the software problem --- located in what is known as an electronic control module --- to slow sales of the Prius.
"Demand for vehicles that have lower emissions and high gas mileage will still continue," Takahashi said.
He noted that many cars have one or more electronic control modules and that problems are not uncommon.