In a move which marks the upgrading of the US National Highway Traffic Safety Administration (NHTSA)'s May 2010-launched Ford defect investigation to an engineering analysis, the auto safety regulator has expanded the probe into customers' complaints that the floor mats in Ford vehicles can trap accelerators; thereby causing a potential risk of a crash.

With the engineering analysis by the NHTSA being the most serious level of inquiry by the agency, the scope of the investigation has now broadened to nearly two-fold more Ford vehicles than the number of vehicles already being investigated for defects.

The Ford vehicles which are now being reviewed by the NHTSA include the 2008 through 2010 models of Ford Fusion, Mercury Milan and Lincoln MKZ sedans. Earlier, the vehicle under review included only the 2010 Fusion and Milan models. In all, an estimated total of 480,000 Ford vehicles are covered in the scope of the auto safety regulator's investigation.

About the issue pertaining to floor mats, the NHTSA said in a December 12 filing that it has reviewed 52 driver complaints about the problem linked to unintended acceleration caused by sticking floor mats.

Revealing that 41 complaints have been submitted by Ford as part of the inquiry, the NHTSA said in the filing: "The accelerator pedal may fail to return to idle due to interference created by unsecured or double stacked floor mats in the driver's foot-well."