Seven crashes have been reported because of the problem, including one in which a driver put a car in park, got out and was struck and injured when the car unexpectedly rolled backward, the National Highway Traffic Safety Administration said Monday in documents posted on its website.
Another car put in park with the engine running reportedly accelerated and ran into a wall after the driver got out.
NHTSA said the protective jacket around cables connecting the floor shift lever to the transmission can deteriorate, exposing the cables to the elements. Cables can corrode and weaken, and the shift lever position can change so it doesn't match the car's gear. That means a driver could put the car in park but the transmission could be in drive or reverse. In some cases, the car may not shift at all.
The probe began with complaints about the Saturn Aura midsize car from the 2007 and 2008 model years, affecting nearly 89,000 vehicles. But the agency said it is now checking to see if the problems extend to other GM vehicles. The Chevrolet Malibu from the 2004 to 2008 model years and the Pontiac G6 from the 2005 to 2008 model years have similar shifting systems, the agency said.
NHTSA said it received three reports of crashes caused by the problem, while GM received four, including the one that injured the driver. No details were given on whether the driver was seriously hurt.
In another case, a driver put the car in park, but the transmission stayed in reverse. "When the driver exited with the engine running, the vehicle continued (accelerated) in reverse and struck a building," NHTSA said.
GM told the agency that the problem affects mainly models with four-speed transmissions, but not six-speed transmissions.
NHTSA said it has upgraded the investigation to an analysis that will check how many times the cables have failed, the consequences "and the scope of vehicles that may be affected, including the models, model years and transmission types."
An investigation doesn't necessarily mean that the vehicles will be recalled.
Company spokesman Alan Adler said GM is cooperating in the investigation. The company will make free shifter cable repairs on affected 2007 and 2008 Auras, and it will cover the repairs for 10 years or 120,000 miles from the date the original owner picked up the car. Those who already have paid for repairs should keep receipts for reimbursement, Adler said.
Although the Malibu and G6 have similar shift systems, the problem happens far less frequently on those cars than it does on the Aura, Adler said. The same parts maker supplied cables for all three vehicles, but the Malibu and G6 cables are not exactly like the Aura's, he said.
Anyone who has an issue with shifting should contact a GM dealer. GM got rid of the Saturn and Pontiac brands last year, and other dealers, mainly Chevrolet, became their authorized repair centers.
GM sold more than 815,000 Chevy Malibus in calendar years 2004 through 2008 and over 570,000 Pontiac G6 models from 2005 through 2008, according to Autodata Corp. and Ward's AutoInfoBank. It's unlikely that all of those cars will be covered by the investigation because they have several types of transmissions.
In March of 2009, GM recalled about 277,000 vehicles for a similar shift lever problem, although NHTSA documents say that was caused by a faulty clip used to adjust the cable.
At that time the recall involved 2009 model year versions of the Buick Enclave; Chevrolet Cobalt, HHR, Malibu and Traverse; GMC Acadia; Pontiac G5 and G6; and the Saturn Aura and Outlook. Only 75,000 of the vehicles had been sold, and the rest were still at dealerships.
Shares of General Motors Co. rose 41 cents, or 1.8 percent, to $22.91 in afternoon trading.