In the wake of a massive public-relations nightmare involving brake problems in its cars, Toyota is investigating two more reports this week of unintended acceleration in its vehicles. Both cases involved Priuses: one in Harrison, NY, that resulted in a crash, and the other on an interstate east of San Diego.

The troubled carmaker has already recalled 5.4 million cars. It has replaced floor mats that it said snagged acceleration pedals, and it has modified certain pedals that the company said could be prone to sticking. Toyota also recently announced that it would retrofit some models with a brake-override system, also known as a "smart pedal."

It rejects claims that the car's electronic throttle system could be responsible for unwanted acceleration.

On February 23, David Gilbert, a professor of automotive technology at Southern Illinois University Carbondale, told a congressional hearing that he was able to short-circuit the electronics behind the gas pedal in a Toyota Tundra, causing it to accelerate automatically without triggering a fail-safe system. Gilbert's review was commissioned by Safety Research & Strategies, a company hired by lawyers representing people who allege that unintended acceleration in Toyota cars has caused accidents.

Toyota hired the consulting firm Exponent to test Gilbert's claims. On Tuesday, the company presented the results of its own experiments, which it says show that the electronic system in question needs to be modified significantly to malfunction in the way described. Christian Gerdes, a professor of mechanical engineering at Stanford University's Center for Automotive Research who took part in the experiments, says that Gilbert "created a different pedal circuit than the one Toyota engineered," and that it doesn't accurately reflect a real-world situation. (Stanford's Center for Automotive Research is affiliated with Toyota.)

"We haven't found any problems with the electronic throttle control system, and we have thoroughly and rigorously tested it many times on vehicles that have had complaints," adds Brian Lyons, a safety manager at Toyota. Nonetheless, Lyons says, a brake-override system will be installed on every Toyota in North America by the end of the year and globally by the end of 2011.

The brake-override feature, already found in many cars, is a software program that monitors sensors in a vehicle's gas and brake pedals. If both pedals are pressed while the car is moving at a certain speed, the program gives precedence to the brake by cutting off the engine. Lyons says that Toyota has been developing a smart pedal for some time. "It was designed to be a feature that would be included in vehicles in the future," he says.