Toyota Sienna Service Manual: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

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

HINT: This DTC relates to the thermostat.

DESCRIPTION This DTC is set when the Engine Coolant Temperature (ECT) does not reach 75C (167F) despite sufficient engine warm-up time

MONITOR DESCRIPTION

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

MONITOR STRATEGY

TYPICAL ENABLING CONDITIONS

TYPICAL MALFUNCTION THRESHOLDS

INSPECTION PROCEDURE

HINT: Read freeze frame data using the intelligent tester. The ECM records vehicle and driving condition information as freeze frame data the moment a DTC is stored. When troubleshooting, freeze frame data can be helpful in determining whether the vehicle was running or stopped, whether the engine was warmed up or not, whether the air-fuel ratio was lean or rich, as well as other data recorded at the time of a malfunction

1 CHECK ANY OTHER DTCS OUTPUT (IN ADDITION TO DTC P0128)

1. Connect the intelligent tester to the DLC3.
2.  Turn the ignition switch to the ON position.
3.  Turn the tester on.
4.  Select the following menu items: DIAGNOSIS / ENHANCED OBD II / DTC INFO / CURRENT CODES.
5.  Read the DTCs.

Result

HINT: If any DTCs other than P0128 are output, troubleshoot those DTCs first.

2 CHECK COOLING SYSTEM

1. Check for defects in the cooling system that might cause the system to be too cold, such as abnormal radiator fan operation or any modifications.

3 INSPECT THERMOSTAT

1. Remove the thermostat.
2.  Check the valve opening temperature of the thermostat.

Standard: 80 to 84C (176 to 183F)

HINT: In addition to the above check, confirm that the valve is completely closed when the temperature is below the standard.

3.  Reinstall the thermostat

REPLACE ECM