The company said Friday it has received no reports of the crack occurring in field and no injuries have been reported.
The voluntary recall affects seats with model numbers starting with 310. They were built from Nov. 24, 2009 to April 9 of this year. Another 4,479 seats are being recalled in Canada. They were made through April 26 of this year.
The company says it will send notices to registered owners of the seats. Evenflo plans to distribute an easily installed reinforcing bracket that will fix the problem.
Consumer Reports magazine said the recall came after its tests showed the seat can crack and fail in a simulated 30 mph frontal collision.
Children weighing under 40 pounds should not use the seat without the repair bracket, Evenflo said in a statement. Those over 40 pounds can still use it as a seat belt-positioning booster until the repair kit arrives, the company said.
Evenflo said it will remove all the affected seats from store shelves.
Model numbers of the seats can be found on a white label behind the seat above the highest shoulder belt slot, the company said. It said other Maestro models are not affected.
Seat owners can go to the company's website, www.evenflo.com, for more information. They can call 1-800-233-5921 between 8 am and 5 p.m. EDT to get a free reinforcement kit.
Consumer Reports said the Maestro, which costs about $80, can be used with its own five-point harness as a conventional child safety seat, or it can be used by larger children as a booster seat anchored by a car's seat belt.
An outside laboratory hired by the magazine found that the plastic shell cracked and the harness was loosened in two seats during a simulated crash. Both failures came when the seat was used with its own five-point harness, the magazine said.