The issue focused largely on auto manufacturers such as Chrysler Group LLC and Fiat Chrysler Automobiles NV (FCAU), whose executives attended Tuesday's forum with VW AG chairman Matthias Mueller. The CEOs urged Volkswagen to comply with emissions regulations designed to curb greenhouse gases from automobiles.

VW engineers argued they didn't believe cars emitted pollutants enough because vehicles drove relatively lightly during peak periods when diesel engines exhaust fumes into air pollution monitors that sensors generate readings for emission monitoring devices. Analysts noted those monitors emit measurements within limits determined by