The software sensed when the car was being tested and then activated equipment that reduced emissions, United States officials said. But the software turned the equipment off during regular driving, increasing emissions far above legal limits, possibly to save fuel or to improve the car’s torque and acceleration.It is not yet known which systems were modified. But experts are focusing on parts of the exhaust system that are designed to reduce emissions of nitrogen oxide, a pollutant that can causeemphysema, bronchitis and other respiratory diseases. RELATED ARTICLE »