Chevrolet Repair Questions? Ask a Chevy Mechanic for Answers ASAP
The most common fail item associated with the P0128 is a faulty ECT (Engine Coolant Temp) Sensor.
If you can double check your engine size (Impala was not offered with a 3.5L in 2005. Either 3.4L or 3.8L) I can get you a location and replacement procedure.