What is error code p0128 in Chevy Impala?

Answer 1

P0128: Engine Coolant Temperature is below the Thermostat Regulating Temperature

Basically the Computer sees that the engine is running too cold.

It could be that the thermostat is stuck open and the engine is actually too cold.

Or it could be a bad temperature sensor, wiring to the sensor, or the PCM (computer) has a problem and is making a mistake.

I would probably replace the thermostat and ECT (Engine Coolant Temp) Sensor first because both are inexpensive and relatively easy to change.