Circuit Description

WARNING: This page is about a different car, the 2003 Lexus RX 300. However, it is still accessible from the selected car via links, so may be relevant.

The ECT sensor is a thermistor that monitors coolant temperature. DTC P0116 sets when THW is less than 95°F (35°C) and 140°F (60°C) , THA is greater than 19.4°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 when starting engine. In case 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 THW more than 140°F (60°C) when starting engine.

DTC P0125 sets if THW is less than 20°F (-6.7°C) or THA is less than 20°F (-6.7°C) 20 minutes or more after starting engine and engine coolant temperature sensor value is 68°F (20°C) or less (2 trip detection logic), if THW is greater than 20°F (-6.7°C) and THA is greater than 20°F (-6.7°C) and 50°F (10°C) at engine start, 5 minutes or more after starting engine, engine coolant temperature sensor value is 48°F (20°C) or less (2 trip detection logic), or if THW is greater than 50°F (10°C) and THA is greater than 50°F (10°C) at engine start, 2 minutes or more after starting engine, engine coolant temperature sensor value is 48°F (20°C) or less (2 trip detection logic) mode, in which engine coolant is assumed to be 176°F (80°C). Possible causes are:

Faulty ECT sensor.
Faulty cooling system.

NOTE: If DTCs P0115 and P0116 are output together, diagnose DTC P0115 first. Inspect engine coolant temperature circuit for an open. Repair as necessary. See ENGINE PERFORMANCE in WIRING DIAGRAMS article in ELECTRICAL.