P0128
DTC P0128 COOLANT THERMOSTAT (COOLANT TEMP. BELOW THERMOSTAT REGULATING TEMP.)Component Location:
GENERAL DESCRIPTION
An Engine Coolant Temperature Sensor (ECTS) monitors the temperature of the coolant. This input is used by the ECM for engine control and as an enabling criteria for related diagnostics. The air flow coming into the engine is accumulated and used to determine if the engine has been driven within conditions that would allow the engine coolant to heat up normally to the thermostat regulating temperature. If the coolant temperature does not reach regulating temperature of the thermostat, diagnostics that use engine coolant temperature as enabling criteria, may not run when expected. This DTC will set when there has been excessive time to reach a minimum coolant temperature required for closed loop fuel control.
DTC DESCRIPTION
An open stuck thermostat means an increase of the engine warm up time and can cause emission increase as well. To detect open stuck thermostat, the ECM checks measured coolant temperature as soon as calculated coolant temperature reaches threshold and sets DTC P0128 when measured coolant temperature is lower than threshold. If same code is set again in the next driving cycle, MIL is illuminated.
DTC Detecting Condition:
Specification:
Schematic Diagram:
MONITOR DTC STATUS
NOTE: If any codes relating to ECTs are stored, do ALL REPAIRS associated with those codes before proceeding with this troubleshooting.
1. Connect scan tool and select "Diagnostic Trouble Codes (DTCs)" mode.
2. Press F4 (DTAL) to select DTC information from the DTCs menu.
3. Confirm that "DTC Readiness Flag" indicates "Complete". If not, drive the vehicle within conditions noted in the freeze frame data or enable conditions.
4. Read "DTC Status" parameter
5. Is parameter displayed "History (Not Present) fault"?
NOTE:
- History (Not Present) fault: DTC occurred but has been cleared.
- Present fault: DTC is occurring at present time.
YES - Fault is intermittent caused by poor contact in the sensor's and/or ECM's connector or was repaired and ECM memory was not cleared. Thoroughly check connectors for looseness, poor connection, bending, corrosion, contamination, deterioration, or damage. Repair or replace as necessary and then go to "Verification of Vehicle Repair" procedure.
NO - Go to next step as below.
MONITOR SCANTOOL DATA
1. Allow the engine to cool completely
2. Run the cold engine at idle for 5 minutes and observe cooling fan status.
NOTE: Ensure that the A/C is OFF
3. Check the engine coolant temperature parameter at idle with the scantool.
4. Is the engine coolant temperature increase to above 50 °C (122 °F)
YES - Go to "Terminal and Connector Inspection" procedure.
NO - Go to next step as below.
5. Are the cooling fans running when engine coolant temperature is low (less than approximately 98 °C (208 °F)) with A/C OFF?
YES - Check for short circuit in cooling fan harness or cooling fan relay. Repair as necessary and go to "Verification of Vehicle Repair" procedure.
NO
- With ignition "ON", Install scantool and select "COOLING FAN RELAY" on the Actuation Test mode.
- Activates "COOLING FAN RELAY" by pressing "STRT (F1)" key
- Repeat this procedure 4 or 5 times to ensure cooling fan reliability.
- If cooling fan works properly, go to next step as below
- If NG, check for intermittent fault caused by poor contact in the sensor's and/or ECM' s connector.
- Repair as necessary and go to "Verification of Vehicle Repair" procedure.
SYSTEM INSPECTION
1. Coolant Level Inspection
1. Check the cooling system coolant level.
2. Is the coolant in the reservoir at the proper level?
YES - Go to next step as below.
NO - Repair or add engine coolant as necessary and go to "Verification of Vehicle Repair" procedure.
2. Thermostat Inspection
1. Check if the thermostat bypass valve is stuck in the open position or if the correct type of thermostat was installed. Replace thermostat as necessary and go to "Verification of Vehicle Repair" procedure
2. Check the valve opening temperature of the thermostat
Specification (Valve opening temperature) : 80 - 84 °C (176 - 183 °F)
3. If the opening temperature is not as specified, Replace thermostat as necessary and go to "Verification of Vehicle Repair" procedure.
If OK, go to next step as below.
3. ECT Sensor Inspection
1. Ignition "OFF"
2. Disconnect ECTS connector
3. Measure resistance between terminals 1 and 3 of the sensor connector (Component side)
Specification:
4. Is resistance within the specification?
YES - Go to next step as below.
NO - Check ECTS for contamination, deterioration, or damage. Substitute with a known-good ECTS and check for proper operation. If the problem is corrected, replace ECTS and then go to "Verification of Vehicle Repair" procedure.
TERMINAL AND CONNECTOR INSPECTION
1. Many malfunctions in the electrical system are caused by poor harness(es) and terminals. Faults can also be caused by interference from other electrical systems, and mechanical or chemical damage.
2. Thoroughly check connectors for looseness, poor connection, bending, corrosion, contamination, deterioration, or damage.
3. Has a problem been found?
YES - Repair as necessary and go to "Verification of Vehicle Repair" procedure.
NO - Go to next step as below.
VERIFICATION OF VEHICLE REPAIR
After a repair, it is essential to verify that the fault has been corrected.
1. Connect scan tool and select "Diagnostic Trouble Codes (DTCs)" mode.
2. Press F4 (DTAL) and confirm that "DTC Readiness Flag" indicates "Complete". If not, drive the vehicle within conditions noted in the freeze frame data or enable conditions.
3. Read "DTC Status" parameter.
4. Is parameter displayed "History (Not Present) fault"?
YES - System performing to specification at this time. Clear the DTC
NO - Go to the applicable troubleshooting procedure.