WARNING: This page is about a different car, the 2003 GMC Envoy XL and 2003 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: DTC P0128: ECT Less Than Thermostat Regulating Temperature (Steps 1-8)
Courtesy of GENERAL MOTORS CORP.
Fig 2: DTC P0128: ECT Less Than Thermostat Regulating Temperature (Steps 9-15)