2006 Chevy Impala reduced engine power

What I would like you to do is call around to your local parts stores and see if one does a free scan. Go there and scan it and write down the codes found. Then clear the codes and get back to me with the codes. Lets see what the car can tell us about what it happening and go from there.

The engine computer has detected a sensor out of range or not reading at all.

Start by taking to most autoparts stores and get the engine computer scanned.

Autoparts stores can only scan "P" (powertrain) codes. If no "P" codes appear, you will have to get it scanned at a trusted garage/dealer.
There are 4 different codes generated, "P", "C" (chassis), "B" (body) and "U" (communications).

Autoparts stores that do scan, do this for free, so free is good :).

After you get a code, post codes and we will go from there.

Newer cars are equipped with a "fly by wire" system, where there is no actual physcial connection to throttle body (which opens and lets more air into engine as throttle pedal is depressed). Older cars have a throttle cable that is a mechanical connection from gas pedal to the throttle plate.

The voltage generated as you depress the throttle is sent to a small motor on the throttle body that opens the plate. This error was generated because the throttle plate was sticking and was not sending the expected voltage reading to the PCM.

The on board computer compares the voltage generated by the gas pedal to the throttle plate position from a closed position.
If there is an error in this comparison (correlation), the PCM will default to a small throttle plate opening, in other words it will not allow the throttle plate to open beyond a certain point to prevent unintended acceleration.

It is probably best that this be corrected by the dealer or whoever you are taking it to, while you would have got the code read for free, this type of error is best repaired by a professional.