But the Japanese automaker is adamant the repair being carried out worldwide isn't another recall. It is calling the move a "customer satisfaction campaign" because the problem doesn't warrant a recall under regulations where the vehicles have been sold, according to Nolasco.
Of the Prius cars that need fixing, 390,000 are in North America and 180,000 are in Japan. The problem affects 70,000 Prius vehicles in Europe.
The problem with the Prius, which is closely associated with Toyota's reputation for innovative technology, is the latest embarrassment for the world's No. 1 automaker, which has recalled more than 11 million vehicles globally since late last year.
The massive recalls cover a wide range of models for various problems, including faulty gas pedals, floor mats that can trap accelerators, defective braking and stalling engines.
The popularity of the Prius in Japan had been one bright spot in Toyota's performance, although worries remain that its image in North America has not recovered from the recalls. The hybrid was Japan's best-selling car for the 18th straight month in October, even after green vehicle subsidies ended.
The Prius switches back and forth between a gasoline engine and an electric motor to deliver better mileage than conventional cars. The latest problem affects the core hybrid system.
Toyota said there have been no reports of accidents related to the coolant problem, but 300 problems were reported in Japan, and 28 outside of Japan. It was not immediately clear when owners were being notified to bring their cars in for repair.
Owners that already had the problem repaired can seek reimbursement from Toyota, Nolasco said.
In the old design, air could get stuck in the coolant pump, causing it to malfunction and risk overheating the hybrid system.
The replacement is designed so that air bubbles won't get trapped inside, Nolasco said.
Toyota is the only major automaker whose U.S. sales have been sluggish despite a recent onslaught of incentives that have boosted other automakers' results -- underlining how its once sterling image continues to be tarnished in that crucial market.
Toyota faces hundreds of lawsuits in the U.S., many from families of people who died or were injured in accidents related to unintended acceleration suspected of being linked to the quality lapses.
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