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Japanese automaker Toyota announced today that it would fix 378,000 of its Prius hybrids built between 2004 and 2007, saying a water pump design defect could produce poor performance.

Toyota Motor Sales national manager of environmental, quality and safety communications John Hanson explained the technical aspect of the problem.

"The water pump is part of a system designed to cool the hybrid powertrain components and is separate from the engine cooling system. A fault in the water pump can cause air bubbles to build up and disrupt the flow of coolant, which can lead to overheating," Hanson said.

While safety systems will automatically activate before the heat buildup becomes sufficient to seriously damage the vital hybrid components, the company said the fail-safe mode will result in reduced performance and power output. The problem could also activate a Prius' check engine light.

Toyota was also careful to note the maintenance notice was not a recall, as the problem has not been linked to any injuries or crashes. The company said it would notify customers whose vehicles may have the faulty pumps in a mass mailing beginning next month, and that it would cover repair costs for maintenance appointments made before November 30, 2013.

In addition, Toyota said, any Prius driver who has already paid for such repairs – or had his or her car insurance pay for such repairs – would be reimbursed. Instructions for this process will be included in the owner letter, according to Toyota.

Though this latest maintenance issue is not a safety recall, the car maker has been under scrutiny lately for several safety issues pertaining to its vehicles.