Circuit Description

WARNING: This page is about a different car, the 2003 Pontiac Vibe. However, it is still accessible from the selected car via links, so may be relevant.

The DTC P0128 Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature diagnostic monitors the relationship between engine temperature and engine running time. ECT sensor input is used to monitor performance of engine's cooling system thermostat. A DTC P0128 may set if ECT sensor voltage does not indicate engine coolant temperature has increased sufficiently during running of the diagnostic. When ambient air temperature and engine operating conditions are right, Powertrain Control Module (PCM) starts the diagnostic using a water temperature counter. Water temperature counter times how long the cooling system takes to reach 167°F (75°C). Failure to reach 167°F (75°C) within the diagnostic window can indicate a malfunctioning thermostat.