P1128 code,they replaced throttle body unit.with their hand held computer it taught part to sync with car computer so it works like it should,it didnt work.my check engine light still comes on and my car shuts down to 5 mph.need them to fix it right.what should i tell my mechanic to do???

First off make sure they did what's called a "throttle valve closed position learning" & an "idle air learning" procedure. The following is the procedures they should have done before going ahead and replacing the throttle valve control motor which I am guessing that's the unit you are talking about.

1. Disconnect and reconnect ground at the motor (picture below of its location)
2. This may sound strange but if you go in there using this wording then it will sound as if you now what you are talking about. Ok, At the throttle control actuator harness they should check terminal 3 to terminal 114 at the ECM. Continuity should not exist. Then check the same terminal to ECM terminal 116. Continuity should exist. Picture of harness below.
3. At the throttle control actuator harness they need to check terminal 6 with ECM terminal 114. Continuity should exist. At the same terminal check it with terminal 116 at the ECM Harness and continuity should not exist. On steps 2 & 3 if the test was no good repairs to the wiring should be done.
4. Next they should have check the unit itself by checking the resistance at the actuator harness between terminals 3 & 6. It should have between 1 & 15 ohms. If not then the unit would be faulty.
5. If all the above tests passed then they need to check the shield current which would be way to much to post here but if that failed repairs to the harness or connector should be done.

All of this should have been done before replacing the unit itself. As you may have guessed I am in this business and come across many shops who have no idea what to test when it comes to Nissans. Nissans are controlled highly by its sensors which in turn tell the computer (ECM) how to make the vehicle run. They are not like many cars to where you get a code and just replace a sensor due to the fact there is a lot of wiring through many connectors which can go bad. If you have any questions or would like me to make you up a file right from the factory service manual by Nissan to take with you, email me at csautomotivecars@att,net. I just hate to see people get taken by shops that don't know what they are dealing with. I hope this wasn't to technical for you and I hope this helps. Good luck. Here are your pictures: