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updated 8/24/2010 4:28:59 PM ET 2010-08-24T20:28:59

U.S. safety regulators have stepped up their investigation into complaints of stalled engines on Toyota Corolla sedans and Matrix hatchbacks, but government officials stopped short of announcing a recall.

The National Highway Traffic Safety Administration upgraded its investigation last week and is looking at two possible causes behind the stalling: Improper coating on circuit boards and a crack in the surface of a glass coating. Both could have happened during production of the cars.

The new probe, called an engineering analysis, covers 1.8 million Corolla and Matrix cars from model years 2005 to 2007. It is a more in-depth investigation of the vehicles than the preliminary probe begun by NHTSA in December. That probe covered vehicles from the 2006 model year only.

NHTSA said it has received 163 complaints of stalling engines in the cars as of last week.

Toyota spokeswoman Monika Saito in Tokyo confirmed the NHTSA investigation. She said Toyota was cooperating fully but declined to give more details.

It was unclear whether a recall will be necessary, though the engineering analysis will determine if there is a safety defect that needs to be fixed.

A problem with the Corolla, one of Toyota's top-selling models, would further embarrass the Japanese automaker, which has long prided itself on its quality controls. The Matrix is the hatchback version of the Corolla.

The automaker is trying to patch up its reputation following the recall of some 10 million cars and trucks around the world since October 2009 over safety problems, including brake glitches, defective gas pedals and floor mats that trap accelerators.

Copyright 2010 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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