Julie Dean, 53, died after she was hit by a Qashqai in Leyland, Lancashire in July 2014.
Driver Ann Diggles, now 82, was trying to park and denies causing the death by dangerous or careless driving.
At Preston Crown Court, she said the car "took off" because, her lawyers said, of a fault with the vehicle.
Her lawyers suggested there was a problem with the electronic throttle system, which resulted in an "uncommanded acceleration".
But prosecutors argued that no problem was found with the throttle.
Takuma Nakamura, who is responsible for engine control systems development at Nissan, was asked by prosecutor Richard Archer: "Is it possible, in your opinion, for a malfunction in an electronic throttle to cause sudden acceleration of the vehicle?"
Mr Nakamura replied: "I think that's impossible."
He said the system, in which the computer rather than the driver controls the throttle opening settings, had a self-diagnostic feature and that any problem would have been recorded.
Mr Nakamura also discounted the defence's theory that an undercharged battery could have led to the malfunction and said the vehicle could not accelerate in those circumstances.
Asked about why the vehicle moved forward before the crash, he said: "In my opinion, for some reason the accelerator has been stepped on."
He also disagreed that short-term "voltage glitches" could cause the vehicle electronics to malfunction.
The jury heard the "low speed" crash happened as Mrs Dean stepped out of a charity shop. She ended up underneath the vehicle and was pronounced dead at the scene.
The court heard Mrs Diggles' car was registered in 2007 and she had driven it for nearly seven years.
The vehicle's original battery was in place and the car had been fully serviced in line with the manufacturer's recommendations.
The trial continues.
A senior manager at Nissan has told a court it was "impossible" that a malfunction caused a Qashqai vehicle to surge forward and kill a pedestrian.