Toyota is expanding a recall of Prius hybrid models over a hybrid-system defect that could cause a sudden loss of propulsion.

The fault lies with the inverter in the Intelligent Power Module (IPM), a part of the Toyota Hybrid Synergy Drive system. Repeated driving under high-load conditions, such as hard acceleration from a stop, can cause high thermal stress in certain transistors within the IPM, according to the National Highway Traffic Safety Administration (NHTSA). This can lead to unusually high voltage buildup, exceeding the limits of software and circuits, the agency said.

This can trigger a fail-safe mode that reduces power, but can also cause a complete loss of propulsion while driving, according to the NHTSA.

The fault was previously identified in another recall, but the 266,368 cars in this recall were not included because they were originally equipped with a different version of the relevant software.

The latest recall includes Prius hatchbacks from model years 2013 to 2015, and Prius V wagons from model years 2014 to 2017. However, Toyota isn't sure how many vehicles actually have the defect.

"Toyota is unable to provide an estimate of the percentage of vehicles to actually contain the defect," the NHTSA recall report said. "Whether the issue in each case will lead to damage of the transistor within the inverter assembly and subsequently lead to a shutdown of the hybrid system, creating an unreasonable risk to safety, depends on each vehicle’s operating conditions. "

2017 Toyota Prius V

Toyota dealers will perform a software update of the hybrid system, and replace inverter assemblies that have failed due to the defect. All work will be performed free of charge.

The inverter fault has been a rare, high-profile failure case for the Prius, which has had core powertrain components that have a stellar reputation for reliability and longevity. Toyota first issued an inverter-related recall in 2014.

Some owners alleged that a previous software fix to remedy the inverter issue in older Prius models decreased mpg.

While these issues don't affect the current generation of the Prius, it hasn't been completely immune to recalls—a brake-booster recall last year affected a few thousand vehicles.

This latest recall is expected to begin August 10. In the meantime, owners can contact Toyota customer service 1-888-270-9371 and reference recall number 20TB10/20TA10. Alternatively, owners can call the NHTSA's Vehicle Safety Hotline at 1-888-327-4236 or visit the agency's safercar.gov website.