DTC P0128-00: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature); DTC P0128-07: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) Mechanical Failure [12/2023 - ]: Description

WARNING: This page is about a different car, the 2024 Lexus TX 550h+. However, it is still accessible from the selected car via links, so may be relevant.

The ECM uses the engine coolant temperature sensor, installed to the water outlet, to monitor the operation of the thermostat.

DTC No.	Detection Item	DTC Detection Condition	Trouble Area	MIL	DTC Output from	Priority	Note
P0128-00	Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)	All of the following conditions are met for 5 seconds or more (2 trip detection logic): Cold start. The engine is warmed up. The engine coolant temperature is less than 75°C (167°F).	Water inlet with thermostat sub-assembly Engine cooling system Engine coolant temperature sensor ECM	Comes on	Engine	B	SAE Code: P0128
P0128-07	Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) Mechanical Failure	All of the following conditions are met for 3 times (2 trip detection logic): The engine coolant temperature is less than 75°C (167°F). The value of the change in engine coolant temperature is less than the threshold.	Water inlet with thermostat sub-assembly Engine cooling system Engine coolant temperature sensor ECM	Comes on	Engine	B	SAE Code: P0128