UPDATE 10:07 ET on 8/27: GM says shutdown for Impala tooling, not Volt.

General Motors is shutting down, for a month, the plant that makes its well-known Chevrolet Volt extended-range electric car. But GM says it is for retooling to produce the coming new Impala, and not, as some reports indicated, to cut back this year's Volt production.

Automotive News, citing unnamed sources, reported the Detroit-Hamtramck plant will suspend production from Sept. 17 until Oct. 15. Leaders of the UAW told the plant's 1,500 union workers about the scheduled downtime last week, the source said.

It would be the second interruption in Volt production this year. GM late Monday emphasized that the action is to prepare the plant to build the redesigned Impala starting in the first quarter.

"We are not idling the plant due to poor Volt sales. We're gearing up for production of the new Impala," Chevy spokesman David Darovitz said in an email. The 2014 Impala, a complete redesign, is a key model for Chevy, and GM wants to take extra time preparing the factory to ensure a high-quality launch, Darovitz said.

Volt sales this year are 10,666 through July, way up from the period a year ago in which GM sold 2,870 of the cars,which can go 38 miles on battery power before needing charging from its gasoline engine. Production was suspended last spring when inventories increased well ahead of demand, but the supply on dealer lots has since been cut almost in half to a more acceptable 85 days worth of cars, according to Auto News.

The Detroit Hamtramck plant (so-called because it straddles the line between the two cities) also builds some of the new-design, 2013 Chevy Malibu sedans, though the primary production is at GM's Fairfax, Kan., factory.

"We are comfortable with our current inventory levels of (Volt and Malibu), which allows us to take time for launch readiness of the Impala. This may include making certain areas of the line more efficient, address space constraints for incoming materials, provide additional training and more," Darovitz said in the email.