Troubleshooting

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

CIRCUIT DESCRIPTION

DTC Detecting Condition:

If the water temperature becomes 75 °C (167 °F) or more, it is abnormal.
If not due to the thermostat malfunction, it is abnormal.

MONITOR DESCRIPTION

Monitor Description:

Monitor Strategy:

Typical Enabling Conditions:

Typical Malfunction Thresholds:

Monitor Result (Mode 06):

The ECM estimates the coolant temperature based on starting temperature, engine loads, and engine speeds. The ECM then compares the estimated temperature with the actual ECT. When the estimated coolant temperature reaches 75 °C (167 °F), the ECM checks the actual ECT. If the actual ECT is less than 75 °C (167 °F), the ECM will interpret this as a fault in the thermostat or engine cooling system or thermostat and set a DTC.

Step 1 - 2:

INSPECTION PROCEDURE

HINT: Read freeze frame data using OBD II scan tool or hand-held tester. Because freeze frame records the engine conditions when the malfunction is detected. When troubleshooting, it is useful for determining whether the vehicle was running or stopped, the engine was warmed up or not, the air-fuel ratio was lean or rich, etc. at the time of the malfunction.