First make sure you have 12 volts to the aftertreatment sensors. I like to test it with a light bulb even if the meter says there is 12 volts. The data link wires, measure resistance between them with the key turned off, should be close to 60 ohms. I don't recall where the terminating resistor is at the aftertreatment module. It may be in one of the sensor modules, if you unplug the module with the terminating resistor the meter will read 120 ohms. The terminating resistor at the front of the data link is inside the turbo actuator. Then with the key on measure the voltage at each wire to a good ground. You should have close to 2.5 volts with one being slightly higher than the other. Broken wires near the aftertreatment assy are common.