For a complete wiring diagram Refer to Section 8W.The engine coolant temperature sensor is a negative temperature coefficient thermistor-type sensor whose resistance varies inversely with temperature. At cold temperatures the sensor resistance is high so the voltage is high. As the coolant temperature increases the resistance decreases and the voltage becomes low. The INSUFFICIENT COOLANT TEMP FOR CLOSED-LOOP FUEL CONTROL determines if the engine coolant temperature will reach the closed loop fueling control temperature limit in a regulated time after start.When Monitored:With battery voltage greater than 10.4 volts and after engine is started.Set Condition:The engine temperature does not go above -10°C (15°F). Failure time depends on start-up coolant temperature and ambient temperature. (i.e. 2 minutes for a start temp of -10°C (15°F) or up to 10 minutes for a vehicle with a start-up temp of -28°C (5°F). Two Trip Fault. Three good trips to turn off the MIL.Possible CausesLOW COOLANT LEVELTHERMOSTATECT SENSORAlways perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE - DIAGNOSIS AND TESTING).1. CHECKING COOLANT LEVEL AND CONDITIONIgnition on, engine not running.With a scan tool, select View DTCs.If an Engine Coolant Temperature (ECT) DTC is set along with this code, diagnose the ECT DTC first.Inspect the ECT terminals and related PCM terminals. Make sure the terminals are free from corrosion and damage.The best way to diagnose this DTC is to allow the vehicle to sit overnight outside in order to have a totally cold soaked engine.Extremely cold outside ambient temperatures may have caused this DTC to set.Never open the cooling system when the engine is hot. The system is under pressure. Extreme burns or scalding may result. Failure to follow these instructions can result in personal injury or death. Allow the engine to cool before opening the cooling system.Inspect the coolant system for proper level and condition.Is the coolant level and condition OK?YesGo To 2NoInspect the vehicle for a coolant leak, make the appropriate repairs, and add the correct amount of coolant.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE) 2. THERMOSTAT OPERATIONThis test works best if performed on a cold engine (cold soak)Ignition on, engine not running.With a scan tool, read the Eng Coolant Tmp Deg value. If the engine was allowed to sit overnight (cold soak), the temperature value should be a sensible value that is somewhere close to the ambient temperature.If engine coolant temperature is above 82°C (180°F), allow the engine to cool until 65°C (150°F) is reached.Start the Engine.During engine warm-up monitor the Eng Coolant Tmp Deg value. The temp deg value change should be a smooth transition from start up to normal operating temp 82°C (180°F). Also monitor the actual coolant temperature with a thermometer.As the engine warms up to operating temperature, the actual coolant temperature (thermometer reading) and the scan tool Eng Coolant Tmp Deg values should stay relatively close to each other.Using the appropriate service information, determine the proper opening temperature of the thermostat.Did the thermostat open at the proper temperature?YesGo To 3NoReplace the thermostat.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE) 3. ECT SENSOR OPERATIONIgnition on, engine not running.With a scan tool, read the Eng Coolant Tmp Deg value. If the engine was allowed to sit overnight (cold soak), the temperature value should be a sensible value that is somewhere close to the ambient temperature.If engine coolant temperature is above 82°C (180°F), allow the engine to cool until 65°C (150°F) is reached.Start the Engine.During engine warm-up monitor the Eng Coolant Tmp Deg value. The temp deg value change should be a smooth transition from start up to normal operating temp 82°C (180°F). Also monitor the actual coolant temperature with a thermometer.As the engine warms up to operating temperature, the actual coolant temperature (thermometer reading) and the scan tools Eng Coolant Tmp Deg value should stay relatively close to each other.Is the thermometer reading relatively close to the scan tool ECT reading?YesTest Complete.NoReplace the Engine Coolant Temperature Sensor.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)