P0128

DTC P0128

Circuit Description

An engine coolant temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the engine control module (ECM) for engine control and as an enabling criteria for other diagnostics. This input also determines if the ECT is insufficient for thermostat regulating of the coolant.

This engine coolant thermostat monitoring diagnostic is designed to detect a stuck open thermostat. When the engine is started the ECM uses a timer and a modeled ECT table that is based on engine coolant temperature at start-up, intake air temperature (IAT), engine speed (RPM) and engine load to determine when the calibrated threshold of 90�C (194�F) has been met. When this threshold is met, the modeled ECT is compared to the actual measured ECT.

If the ECM detects that the modeled ECT has met the threshold of90�C (194�F) and the measured ECT has not met the minimum calibrated thermostat regulating temperature of 72�C (161�F) for more than 10 seconds, DTC P0128 sets.

Conditions for Running the DTC

^ DTCs, P0068, P0101, P0102, P0103, P0112, P0113, P0116, P0117, P0118, P0125, P1101, P1258 are not set.
^ The ECT Sensor Start-Up parameter is between -45 and +42�C (-49 and +107�F)
^ The Intake Air Temperature Sensor parameter is more than -8.25�C (+17�F).
^ The engine idle time is less than 95 percent.
^ The fuel cut off time is less than 50 percent.
^ The engine metal overtemperature protection (EMOP) is not active.
^ This diagnostic will run under normal driving conditions within the enabling conditions.

Conditions for Setting the DTC

If the ECM detects that the modeled ECT has met the threshold of 90�C (194�F) and the measured ECT has not met the minimum calibrated thermostat regulating temperature of 72�C (161�F) for more than 10 seconds, DTC P0128 sets.

Action Taken When the DTC Sets

^ The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
^ The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC

^ The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
^ A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
^ A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
^ Clear the MIL and the DTC with a scan tool.