As Kevin indicated, it's the ol' throttle "snap test" done in this state and is pretty forgiving. Unless your truck has visible smoke coming from the tailpipe, you'll pass with flying colors. They don't measure the chemical makeup of diesel exhaust in this state... only the percentage obscured from smoke.then you have to quickly take the engine up to a high (~2500) RPM and then let off. You do this three time in a row.