Troubleshooting

P0128 - Thermostat Malfunction

CIRCUIT DESCRIPTION

DTC Detecting Condition:

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

Step 1 - 2:

INSPECTION PROCEDURE
Read freeze frame data using LEXUS 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.