If there are no coolant leaks, replace the thermostat first, if the problem is still there check/replace the coolant temperature sensor.
There is a factory service bulletin for the 2005 Toyota Prius:
2005 Toyota Prius Factory Service Bulletin OBDII Code P0125
Check the 'Possible Causes' listed above. Visually inspect the related wiring harness and connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.
The cost to diagnose the P0125 2005 Toyota Prius code is 1.0 hour of labor. The diagnosis time and labor rates at auto repair shops vary depending on the location, make and model of the vehicle, and even the engine type. Most auto repair shops charge between $75 and $150 per hour.
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.
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.
Need more information on how to fix the P0125 2005 Toyota Prius code? Get Access to Factory Service Manuals
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