Circuit Description

An engine coolant temperature (ECT) sensor monitors temperature of coolant. This input is used by engine control module (ECM) for engine control and as enabling criteria for some diagnostics. Air flow coming into engine is accumulated and used to determine if engine has been driven within conditions that would allow engine coolant to heat up normally to thermostat regulating temperature. If coolant temperature does not increase normally or does not reach regulating temperature of thermostat, diagnostics that use engine coolant temperature as enabling criteria, may not when expected. If engine coolant fails to reach a preset target temperature before a calculated air flow is accumulated, DTC P0128 will be set.

Detecting conditions for running DTC are:

• The ECT is between 14°F (-10°C) and 95°F (35°C).
• The Intake Air Temperature (IAT) is between 14°F (-10°C) and 95°F (35°C).
• Difference between ECT and IAT is between 5°F (-15°C) and 13°F (7°C).

Detecting condition for setting DTC is when ECT failed to reach at least 167°F (75°C) during running of coolant temperature counter diagnostic (2 trip detection logic).

Vehicle Response To DTC Set:  The ECM illuminates Malfunction Indicator Light (MIL) on 2nd consecutive ignition cycle that diagnostic runs and fails. ECM records operating 1st time diagnostic fails, ECM stores this information in failure records. If diagnostic reports a failure on 2nd consecutive ignition cycle, ECM records operating conditions of time of failure. ECM writes operating conditions to freeze frame and updates failure records.

Conditions For Clearing MIL/DTC:  The MIL turns off after 3 consecutively passing trips without a fault. A history DTC clears after 40 consecutive warm-up cycles without a fault.