Troubleshooting
DTC P0128 THERMOSTAT MALFUNCTIONCIRCUIT DESCRIPTION
An Engine Coolant Temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the Engine Control Module (ECM) for engine control and as enabling criteria for some diagnostics. The airflow 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 dose not increase normally or does not reach regulating temperature of the thermostat, diagnostics that use the engine coolant temperature as enabling criteria, may not when expected. It engine coolant fails to reach a preset target temperature before a calculated airflow is accumulated, DTC P0128 will be set.
VEHICLE RESPONSE TO DTC SET
The ECM illuminates the Malfunction Indicator Lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
The ECM records the operating first time the diagnostic fails, the ECM stores this information in the failure records. If the diagnostic reports a failure on the second consecutive ignition cycle, the ECM records the operating conditions at the time of the failure. The ECM writes the operating conditions to the Freeze Frame and updates the failure records.
CONDITIONS FOR CLEARING THE MIL/DTC
DTC Detecting Condition:
The MIL turns off after 3 consecutively passing trips without a fault. A history DTC clears after 40 consecutive warm-up cycles without a fault.
Step 1:
Step 2:
INSPECTION PROCEDURE
HINT:
- If DTC P0115 or P0116 is output, first troubleshoot DTC P0115 or P0116. If no malfunction is detected, troubleshoot DTC P0128 next.
- Read freeze frame data using TOYOTA hand-held tester or OBD II scan tool. 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.