2006 Impala Reduced Power

We have a 2006 Impala with a 3.9 V. The motor and transmission was
replaced on this vehicle. We are getting a Reduced Power Message. The
code that come up is for the Throttle Body. The throttle body was replaced
with numerous ones, all new. The throttle body is not the issue. The gas
pedal was replaced. The computer was replaced. And the wires from the
throttle body to the computer were checked. What else in in that loop I do not
know.
When you turn the key, you get the Reduced Power before the car even
starts. The car runs very rough. We deem to be getting spark at all the plugs,
but only plugs affect the idle of the car when the wires are removed. Anyone
have any suggestions?