I have friends with some Cummins pickups in the 2007-2012 range. Their choices of emissions tinkering include some unplugging EGR and just living with a constant CEL that is a "soft" malfunction, I.E., it doesn't affect functioning of the vehicle.
My unplugging of the EGR resulted in an immediate CEL when I started the car and a warning message that I was 75 miles away from reduced function of the engine (limited to 65 mph top speed). I assume that if I go past that it will drop to 55 mph, and then down to the 4 mph speed limit that results when you run out of DEF. It's a "hard" malfunction in that it impedes functioning of the vehicle.