James Lentz, Toyota President and COO of Motor Sales in the US, testifies before a House Energy and Commerce Oversight and Investigations Subcommittee hearing on the "Response By Toyota and NHTSA (National Highway Traffic Safety Administration) to Incidents of Sudden Unintended Acceleration" on Capitol Hill in Washington February 23, 2010.

Massive recalls of popular Toyota cars and trucks still may "not totally" solve frightening problems of sudden, unintended acceleration, the company's American sales chief conceded Tuesday, a day before the Japanese president of the world's largest automaker must confront angry US lawmakers.

House members listened in rapt silence Tuesday to the tearful testimony of a woman whose car unaccountably surged to 100 mph, then they pressed US sales chief James Lentz on the company's efforts to find and fix the acceleration problems - actions many suggested were too late and too limited.

Lentz apologized repeatedly for safety defects that led to recalls of some 8.5 million Toyota cars and trucks, and he acknowledged the changes the company is making probably aren't the end of the story.

Putting remaining doubts to rest is of vital importance to millions more Toyota owners in the United States and elsewhere, who have continued to drive but with serious concerns about their cars. Toyota sales have suffered, too, and a small army of dealers showed up on Capitol Hill Tuesday, arguing that this week's high-profile hearings are unfairly targeting their company.

"We are vigilant and we continue to look for potential causes," Lentz told the House Energy and Commerce Committee.

That search had better continue, a number of lawmakers said, openly questioning Toyota's insistence that the problems are mechanical, not linked to the vehicles' sophisticated electronics.

Without a more vigorous investigation of the possibility that electronics are involved, Texas Republican Rep. Joe Barton said of Toyota's probe: "In my opinion, it's a sham."

The US government is pursuing the electronics question, Transportation Secretary Ray LaHood told the panel. "We're going to go into the weeds on that" and come up with answers, LaHood said. He said the company's recalls were important but "we don't maintain that they answer every question."

Lentz's appearance set the stage for Toyota's president - Akio Toyoda, grandson of the company's founder - to apologize in person on Wednesday.

Toyoda will accept "full responsibility" for the halting steps that led to the recall, according to prepared testimony released in advance. He also will offer his condolences over the deaths of four San Diego, Calif., family members in a crash of their Toyota in late August.

"I will do everything in my power to ensure that such a tragedy never happens again," Toyoda will tell the House Government Oversight Committee. "My name is on every car. You have my personal commitment that Toyota will work vigorously and unceasingly to restore the trust of our customers."

"Quite frankly, I fear the pace at which we have grown may have been too quick" and led to safety defects at the heart of the recall, Toyoda says in his prepared testimony.

There were repeated displays of emotion at Tuesday's daylong hearing - both from the Tennessee woman who survived a 2006 sudden acceleration incident when she was unable to control her runaway Lexus and from Lentz himself, who choked up while discussing the death of his own brother more than 20 years ago in a car accident.

"I know what those families go through," Lentz said.

Rhonda Smith, of Sevierville, Tenn., said her Lexus raced out of control to speeds up to 100 miles an hour, and that nothing she did to try to stop it worked - including braking and shifting into neutral. "I prayed to God to help me," she said, fighting back tears.

"After six miles, God intervened" and slowed the car, Smith said. She said she was finally able to pull off the road onto a median and turn off the engine. She said it took a long time for Toyota to respond to her complaints and even then it was dismissive.

"Shame on you, Toyota, for being so greedy," Smith said as Lentz sat grim-faced with other Toyota officials in the first row of the committee room awaiting his turn to testify. She directed a second "shame on you" at federal highway safety regulators "for not doing your job."

"Listening to Mrs. Smith, I'm embarrassed for what happened," Lentz said. Pressed by committee members as to why Toyota had not had its technicians pour over the Smith car to determine what actually caused the malfunction, Lentz said he wasn't sure where the car was now but "We're going to go down and talk to them and get the car so that they feel satisfied. I want her and her husband to feel safe about driving our products."

LaHood, the transportation secretary, told the panel the US government knew the exact whereabouts of the car and would share the information with Toyota. "All of this has been a big wake-up call for Toyota," LaHood said.

Toyota has recalled some 8.5 million vehicles worldwide - more than 6 million in the United States - since last fall because of unintended acceleration problems in multiple models and braking issues in the Prius hybrid. It is also investigating steering concerns in Corollas. People with Toyotas have complained of their vehicles speeding out of control despite efforts to slow down, sometimes resulting in deadly crashes. The government has received complaints of 34 deaths linked to sudden acceleration of Toyota vehicles since 2000.

Congressional panels are asking whether computerized modern automotive electronics designed to make cars more efficient can sometimes make them less safe.

Lentz said that "two specific mechanical causes" were to blame for the sudden accelerations - misplaced floor mats and sticking accelerator pedals. He insisted electronic systems connected to the gas pedal and fuel line were not to blame, based on tests made by the company in the United States and Japan.

Lentz said that, while the company had not expressly ruled out an electronics malfunction, "We have not found a malfunction" in the electronics of any of the cars at issue. He cited "fail-safe mechanisms" in the cars that were designed to shut off or reduce engine power "in the event of a system failure."

But when pressed by Energy and Commerce Committee Chairman Henry Waxman, D-Calif., on whether he could say with certainty that the fixes now being undertaken would completely eliminate the problems, Lentz hesitated a moment and then replied: "Not totally."

Still, he said chances of unintended accelerations would be "very, very slim" once the recalls were complete. Lentz also said Toyota was putting in new controls so brakes would override the gas pedal on almost all of its new vehicles and a majority of its vehicles already on the road.

House investigators who reviewed Toyota's customer call database found that 70 percent of the complaints of sudden acceleration were for vehicles that are not subject to the recalls over floor mats or sticky pedals,

Rep. Bart Stupak, D-Mich., chairman of an Energy and Commerce subcommittee investigating Toyota's recalls, said the company "misled the American public by saying that they and other independent sources had thoroughly analyzed the electronics systems and eliminated electronics as a possible cause of sudden unintended acceleration when, in fact, the only such review was a flawed study conducted by a company retained by Toyota's lawyers."

Tracking down an electrical problem can be far more difficult, expensive and time-consuming than finding a mechanical problem. Electrical problems can have more than one source, and they can come from inside or outside the car. Mechanical problems often leave clues such as physical damage, where electronic troubles can be hidden in software or leave no trace at all.

Stupak suggested a decade-old law intended to give federal regulators more tools to track vehicle safety defects needs to be strengthened.

Lentz told the committee his company had "not lived up to the high standards our customers and the public have come to expect from Toyota."

"Put simply, it has taken us too long to come to grips with a rare but serious set of safety issues, despite all of our good faith efforts," said the president and chief operating officer of Toyota Motor Sales USA. Inc.

Later, outside the briefing room, Lentz was asked by reporters whether anybody should be fired at Toyota.

"Right now I think the main focus is let's get the customers' cars fixed, let's figure out what went wrong. We are going to be a much more transparent company," he said. "We have to go back and regroup top-to-bottom."

He said Toyota had already completed fixes on 800,000 vehicles and that most customers seemed "extremely satisfied."

Meanwhile, more than 100 Toyota dealers lobbied members of Congress Tuesday, questioning the ability of the government to be impartial given its bailouts of General Motors and Chrysler that left it with a majority ownership stake in General Motors and a smaller stake in Chrysler.

"That's hard for me as a citizen to understand why my tax dollars are going in that direction," Paul Atkinson, a Houston-area Toyota dealer, said.