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

TECHNICAL DESCRIPTION
• The ECM <M/T> or PCM <A/T> 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 below)
• 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


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 <M/T> or PCM <A/T> monitors for this condition once during the drive cycle.
OBD-II DRIVE CYCLE PATTERN
Refer to Diagnostic Function - OBD-II Drive Cycle - Pattern 10 .
TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:
• The thermostat is faulty
• ECM failed. <M/T>
• PCM failed. <A/T>