David Shepardson / Detroit News Washington Bureau

Washington -- Federal regulators have stepped up their probe into complaints that the engines in 2005-07 Toyota Corollas and Matrix hatchbacks can stall at any speed, and not restart.

The National Highway Traffic Safety Administration upgraded its review to what it calls an engineering analysis after reviewing more than 1,100 complaints. Some 1.2 million Corolla and Matrix vehicles were produced during those model years.

The engineering analysis is a more in-depth investigation than the preliminary probe begun by NHTSA late last year, which covered just the 2006 model year.

It was expanded to the other model years after Toyota said the possible cause is an engine control unit used in the three model years.

An engineering analysis can lead to a recall but does not always.

Toyota says it has since upgraded the questionable part, and issued service bulletins to dealers to replace the engine control computer if vehicles failed to start or shifted harshly.

The automaker opted not to recall the vehicles, but said it is cooperating with the investigation.

Toyota received its first report of rough shifting in Corolla and Matrix models in November 2005.

In total, the automaker issued four technical service bulletins to dealers from 2005 to 2007 in connection with Corollas that failed to start. The bulletins advised dealers to replace the engine control computer, and said Toyota would pay for the repair if the vehicle had fewer than 80,000 miles.

Toyota faces at least a dozen lawsuits in Pennsylvania, New Jersey, Minnesota, Florida and California over the issue.

In some of the lawsuits, dealerships identified the engine control unit as the source of stalling.

In November 2007, a Louisiana driver complained that the engine computer apparently failed, causing the brakes to lock up and the steering control to fail.

The driver hit a parking sign.

Toyota dismissed the complaint. "Even if the vehicle had stalled, the steering and brakes would still be operative and at the speed of 15-20 mph she should have been able to both steer and brake the vehicle," Toyota said in denying the claim.

The motorist's insurance company filed suit against Toyota seeking to recover $7,700 in medical bills and vehicle repairs.

Toyota estimated no more than 0.8 percent of engine control units -- eight vehicles out of 1,000 -- would fail over a 10-year-period.

The company doesn't believe the few failures constitute a safety defect, but seemed to acknowledge that the flaw can lead to stalling.

"We understand that some customers have been inconvenienced by engine ECU failure, and some have reported engine stalling," Chris Santucci, Toyota's manager for technical and regulator affairs, told NHTSA in a March 2 letter.

"It is our philosophy that when certain types of engine ECU failures occur, engine stall or shutdown is preferable as opposed to allowing the engine to become damaged or dangerous."

Separately, NHTSA is investigating 749,000 2009-2010 Corolla and Matrix vehicles for steering issues.

Toyota has issued two dealer bulletins. NHTSA opened its probe in February and hasn't decided whether to upgrade the investigation.