Modified diesel engines often provide significant fuel economy and horsepower gains, as Sebold points out from his own experience. These gains may also come at the expense of excessive nitrogen oxide emissions. We already know that diesel enforcement at the federal level is getting serious, as a diesel tuner has been jailed for selling the engine computer tuners needed to perform these conversions. If you have a diesel that has been illegally modified in your state, it's probably time to start looking for solutions.