Possible causes

– Low engine coolant level
– Leaking or stuck open thermostat
– Faulty Engine Coolant Temperature (ECT) sensor
– Engine Coolant Temperature (ECT) sensor harness is open or shorted
– Engine Coolant Temperature (ECT) sensor electrical circuit poor connection

Tech notes

There are factory service bulletin for the following Toyota models:

2003 Toyota Corolla
2003 Toyota Matrix
2003 Toyota Corolla and Matrix Factory Service Bulletin OBDII Code P0125

2004-2007 Toyota Prius
2007-2007 Toyota Prius Factory Service Bulletin OBDII Code P0125

When is the code detected?

The Engine Control Module (ECM) monitors the ECT signal voltage after engine start-up. If, after sufficient time has passed, the sensor still reports that the engine is not warm enough for closed-loop fuel control, the ECM interprets this as a fault in the sensor or cooling system and sets a Detected Trouble Code (DTC).
Example:
The ECT is 0C (32F) at engine start. After 5 minutes running time, the ECT sensor still indicates that the engine is not warm enough to begin active feedback control of the air-fuel ratio. The ECM interprets this as a fault in the sensor or cooling system and sets a DTC.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P0125 TOYOTA Description

The Engine Coolant Temperature (ECT) sensor is used to monitor the temperature of the engine coolant. The resistance of the sensor varies with the actual coolant temperature. The Engine Control Module (ECM) applies a voltage to the sensor and the varying resistance of the sensor causes the signal voltage to vary.

TOYOTA Technical Service Bulletins for P0125 code