The National Highway Traffic Safety Administration said its investigation was raised to an engineering analysis from a preliminary investigation after General Motors Co (GM.N), which owns the Chevy brand, sent the agency data showing 167 reports and 698 warranty claims relating to the issue.

The investigation was initially opened into more than 309,000 vehicles in February, but the upgraded probe covers 341,786 vehicles, a GM spokesman said.

An engineering analysis is the next step in a process that could lead to a recall, although sometimes NHTSA closes such probes without requiring any action.

GM continues to cooperate with NHTSA, and no injuries or accidents related to the issues have been reported, the spokesman said on Friday. GM is investigating the issue itself, including studying the other vehicles from those model years built on the same platform: the GMC Envoy, Buick Rainier, Saab 9-7x and Isuzu Ascender.

GM no longer builds any of the potentially affected vehicles.

NHTSA said it has received 83 complaints, including 66 alleging the door module had melted or burned, with the other 17 referring to erratic or inoperative window switches. Twenty-eight of the reports NHTSA received have led to driver door fires, some of which allege the vehicle was unattended or not in operation.

REQUEST FOR NISSAN PROBE

In addition, a consumer advocacy group in North Carolina has requested NHTSA open an investigation into Nissan SUVs and pickup trucks from model years 2005 through 2010 for possible transmission cooler leaks that could lead to transmission damage. NHTSA said such a probe would affect 857,432 vehicles.

The North Carolina Consumers Council in a letter to NHTSA requested the defect probe after receiving six complaints about a sudden jerking of vehicles at highway speeds. The nonprofit group said the request covers Pathfinder and Xterra SUVs and Frontier pickup trucks.

NHTSA said it will review the consumer group's petition and related data to assess whether a safety probe is warranted.

Nissan officials said they believe that no safety issue exists for the three models.

"The company is cooperating fully with NHTSA on the matter, but cannot comment further on the status of the inquiry at this time," said a statement from Nissan.

The North Carolina group's director of operations, Matthew Oliver, said in the letter to NHTSA that the complaining consumers related how Nissan dealers diagnosed the problem as a failed transmission fluid cooler in the radiator that allowed coolant to contaminate the automatic transmission fluid, resulting in damaged transmission parts. Consumers said there were no warning signs leading to the problems, according to the group.

The source of the contamination is a cracked automatic transmission fluid line that runs through an engine coolant chamber in the radiator, according to documents filed on the NHTSA website. NHTSA has received 512 consumer complaints regarding the transmission problems between August 2005 and May 2012.

Nissan said that in October 2010, it extended its new vehicle warranties as they apply to radiator assembly for all 2005-2010 Frontiers, Pathfinders and Xterras with automatic transmissions to eight years or 80,000 miles from three years or 36,000 miles.

"On a small percentage of vehicles, an internal crack on the oil cooler tube might occur at higher mileages leading to internal leakage of engine coolant," a spokesman for Nissan said.

Nissan said no injuries or accidents have been reported to them on this issue.

(Reporting by Ben Klayman and Bernie Woodall in Detroit; editing by Matthew Lewis, Sofina Mirza-Reid and Leslie Gevirtz)

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