P0128

DTC P0128: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

TECHNICAL DESCRIPTION
- The ECM or PCM checks the time for the engine coolant temperature to reach the judgment temperature.

DESCRIPTIONS OF MONITOR METHODS
- The thermostat is determined to be stuck open if the engine coolant temperature does not increase to 77 °C (171 °F) within a prescribed length of time after cold-starting the engine.

MONITOR EXECUTION
- Once per driving cycle

MONITOR EXECUTION CONDITIONS (Other monitor and Sensor)

Other Monitor (There is no temporary DTC stored in memory for the item monitored)
- Not applicable

Sensor (The sensor below is determined to be normal)
- Volume airflow sensor
- Intake air temperature sensor
- Engine coolant temperature sensor

DTC SET CONDITIONS

Logic Flow Chart:

Logic Flow Chart

Check Conditions
- Engine coolant temperature is between -10 °C (14 °F) and 77 °C (171 °F) when the engine is started.
- The engine coolant temperature - intake air temperature is 5 °C (9 °F) or less when the engine is started.
- The intake air temperature when the engine is started - intake air temperature is 5 °C (9 °F) or less.
- The volume airflow sensors output frequency is in the low frequency (50 - 100 Hz or less) state for 490 seconds or less.

Judgment Criteria
- The time for the engine coolant temperature to rise to 77 °C (171 °F) takes longer than approximately 10 to 32 minutes.
- The ECM or PCM monitors for this condition once during the drive cycle.

OBD-II DRIVE CYCLE PATTERN
None

TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:)
- The thermostat is faulty
- ECM failed.
- PCM failed.

DIAGNOSIS

STEP 1. Check the cooling system.

Refer to Engine Cooling Diagnosis.

Q: Is the cooling system normal?

YES: Replace the ECM or PCM . Then check that the DTC P0128 does not reset.

NO: Repair it. Then check that the DTC P0128 does not reset.