1998 Dodge Dakota Emissions having problems reading codes

When I go to emmisions to get my computer read, it will not connect to their device (nothing happens when the people connect to the reciever under my dash). When I connect my OBD II reader to my truck, it scans without any problems. I am the second owner and the prevoious owner had swaped in another 3.9L V6 from another Dodge dakota that had been totalled (the newer motor has about 80k miles on it and runs fine, no misses, no weird noises). A couple other pieces of information, my "check engine" light is never on and i'm not having anything other electrical problems.