I took My 2000 Dodge Durango 4.7 engine to get it smog tested in Ca. The tester tried at least 3 times to communicate with my OBD syst. with no success. He then took my truck off the machine and ran another car. It worked perfectly. I don't know what to do now. Any suggestions ?
Welcome to Vehicle Maintenance & Repair.
As you surmise, because the testing rig worked when connected to another vehicle, the test rig is OK. The issue is it won't communicate with your car's OBD system.
OBD sockets and their connected wiring and reporting devices can fail, so you'll need to find the failure, and repair whatever's broken. It might be a physical problem with the OBD socket, or a broken or shorted wire between the socket and something else, or an internal fault in one of the electrical devices in the car which are connected to the OBD system.
The first thing to check is all the vehicle's fuses; the OBD system requires electricity, and checking fuses is a good first step. If you want to go further, take a look at a few of the how-to videos that are easily found online by searching for "OBD socket won't communicate with reader," for example this or this.
If electrical diagnosis and repair make you twitchy, you'll need to find a capable mechanic to find and rectify the problem.