My rough understanding from reading the discussions at the time is that they can pull the engine into a performance range that passes the emissions test, but th

My rough understanding from reading the discussions at the time is that they can pull the engine into a performance range that passes the emissions test, but that range is 1) less horsepower, fewer lb-ft of torque and 2) worse fuel economy. Diesel as a technology suffers from more NOx production than gasoline because the increased compression forces it to run lean rather than at stoichiometric ratios. So, basically they tell the injection software to run in emissions test mode all the time and it's "fixed" but you're not going to get any of the other claimed numbers.