Circuit Description

WARNING: This page is about a different car, the 2003 Lexus GS 430. However, it is still accessible from the selected car via links, so may be relevant.

ECT sensor is a thermistor that monitors coolant temperature. If DTC is set, ECM will operate in fail-safe mode. During fail-safe mode, engine coolant temperature is fixed at 176°F (80°C). DTC P0116 will set when circuit THW is equal to or greater than 95°F (35°C) and less than 140°F (60°C), and circuit THA is greater than or equal to 19.9°F (-6.7°C) when starting engine, vehicle speed is changing (not stable) and water temperature change is lower than 37.4°F (3°C) from water temperature since starting engine, or if reading value of water temperature sensor will not change more than 33.8°F (1°C), even after repeating 6 trips (detection logic) of adjusting speed pattern with circuit THW more than 140°F (60°C) when starting engine. DTC P0125 will set when circuit THW is less than 19.9°F (-6°C) or circuit THA is less than 19.9°F (-6°C), at engine start or 20 minutes or more after starting engine, engine coolant temperature sensor value is 68°F (20°C) or less (2 trip detection logic), if circuit THW is equal to or greater than 19.9°F (6.7°C) and circuit THA is greater than or equal to 19.9°F (6.7°C) and circuit THW is less than 50°F (10°C) at engine start, 5 minutes or more after starting engine, engine coolant temperature sensor value is 68°F (20°C) or less (2 trip detection logic), or if circuit THW is equal to or greater than 50°F (10°C) and circuit THA is greater than or equal to 50°F (10°C) at engine start, 2 minutes or more after starting engine, and engine coolant temperature sensor value is 68°F (20°C) or less (2 trip detection logic). Possible causes are:

Faulty cooling system.
Faulty engine coolant temperature sensor.

NOTE: If DTCs P0115, P0116, P0117, P0118 or P0125 are output simultaneously, engine coolant temperature sensor circuit may be open. Perform troubleshooting for DTC P0115 first.