When Volkswagen personnel were driving one of the 500 test cars made at the Chattanooga plant so far, they uncovered a computer software glitch that caused the sunroof not to close completely.

The VW logo is seen on the front of one of dozens of new Passats made at the Chattanooga Volkswagen assembly plant parked outside the plant. These cars will be used as demos for testing, internal quality control and press test drives.

The discovery prompted VW to change the car's software and ensure that future vehicles won't have that issue.

As VW edges to within a few weeks of assembling customer cars at the factory, the production of test vehicles has helped the company refine the all-new Passat and focus on quality, officials said.

"Quality is No. 1," said company spokesman Scott Wilson.

This week, the German automaker made its 500th test car at the $1 billion plant. But none of those cars ever will go up for sale, according to VW.

"That's not what they're built for," Wilson said. "They'll never be sold or purchased."

At a base price of $20,000 a vehicle, which includes profit, that means VW, in order to ensure quality, will have produced about $10 million worth of cars it will never even try to sell.

Some of the test cars may go to car shows or even into dealer showrooms to be put on display, Wilson said. Others, especially the newer ones, may continue to be driven by VW personnel for added quality evaluation, he said.

The others will be scrapped, Wilson said.

Frank Fischer, chief executive of VW's Chattanooga operation, said recently that, in addition to local testing, the Passat is undergoing drives in extreme conditions in parts of the world such as Northern Europe and the Middle East. He said the road test weather conditions overseas range from below zero to 125 degrees Fahrenheit.

Sometimes, VW board members go on test drives as well to check the midsize sedan's progress, Fischer said.

"What a true pleasure to sit in a car with [Volkswagen AG CEO Martin Winterkorn] and get his instant feedback," he said.

"Get it right"

Fischer said the aim of the test cars and on-the-road workouts is to "really get it right. We really want to make sure we have outstanding quality."

He said a successful launch of the new car is key.

"That's why we've put a lot of effort in it," Fischer said.

Wilson said production of the test cars is typical when VW assembles a brand-new vehicle such as the American-made Passat.

"This happens everywhere," he said. "This is how it has come to be."

In a few weeks, the start of production on customer cars in Chattanooga will mark a milestone for the plant, which started construction in late 2008.

Steve Leach, the city's public works administrator, said the plant's 40-acre outbound loading yard - which will hold the finished cars awaiting shipment across North America by rail or truck - is close to completion.

"They've done 30 percent of the striping (for parking spaces)," he told the city's Industrial Development Board on Tuesday.

City Engineer Bill Payne said there will be more than 3,000 spaces in the yard.

The Chattanooga plant is expected to produce 50,000 cars this year, VW Group of America CEO Jonathan Browning said at the Detroit Auto Show in January. When fully ramped up, the factory is to make 150,000 vehicles a year and employ more than 2,000 people.

Dealership sales are to start in late summer, officials said.

Auto analyst Jesse Toprak of TrueCar.com said he is projecting VW sales in the United States to grow at a stable pace.

"The potential is quite high," he said.

Toprak said he wished VW had been a little more aggressive in the styling of the new Passat, but the automaker has a strong following.

"They expect a tremendous increase in sales in North America," he said. "That may be over optimistic a little."

By 2018, VW has set a target to triple annual sales in the U.S. to 800,000 Volkswagen-brand vehicles and 200,000 Audis.