How to Know if Your Water Valve Sensor is Failing

Like most car parts, water valve sensors eventually fail. In this article, you’ll learn more about water valve sensors, how they fail, the most common symptoms of a failing water valve sensor, and how to diagnose the problem.

What is a Water Valve Sensor?

Valve in car

Image credit: Stomper88

All Toyota Prii have a water valve, which is a cooling system part that controls the coolant flow to three parts: the engine, the heater core, and the coolant heat storage tank. Also called a coolant flow control valve, the water valve has a passage to each part and it works with the ECM to open and close each passage in accordance with the engine.

Inside the water valve is a water valve sensor, which is a small electronic device that communicates with the ECM about the position of the water valve. It exists as a vessel of communication between the ECM and the water valve to ensure that the valve’s working properly with the engine. Read more about water valve sensors here.

Why Do Water Valve Sensors Fail?

The most common reason a water valve in a Prius fails is a malfunctioning radiator. When your Prius has a radiator that isn’t working properly, your engine overheats and wears down the sensor, eventually leading to a shorted water valve sensor circuit. Certain 2004-2009 Prius models are prone to water valve sensor failure, which leads to a sticking water valve.

A sticking water valve causes a disruption in the flow of coolant and can result in serious problems like an overheated engine or a coolant leak. That’s why it’s a good idea to keep an eye out for symptoms of a failing water valve sensor in your Prius.

The Most Common Symptoms of a Failing Water Valve Sensor

The biggest symptom is an illuminated “check engine” light, but it could mean anything. The following symptoms are better indicators that something’s wrong with your water valve sensor.

  • Rough idle
  • Reduced engine power
  • A non-responsive throttle
  • The engine making a thumping or ticking sound
  • The hood feeling hot to the touch
  • The engine smelling “hot”
  • A coolant leak
  • Low coolant levels with no signs of a leak
  • The car not starting properly

How to Diagnose a Failing Water Valve Sensor

If you’re experiencing some of the symptoms of a failing water valve sensor, then we suggest using an OBDII code reader to confirm that your water valve sensor is failing. The OBDII code to look out for is P1121, which means that your water valve is stuck or your water valve sensor circuit is either shorted or corroded.

If you’ve confirmed that you’re dealing with a failing water valve sensor, the only way you can fix this issue is to replace the water valve. You can either order an OEM part from a reputable seller (part no. 16670-21010) and replace it yourself or splurge on a repair job at your local Toyota dealership.

If you need a tutorial on replacing the water valve yourself, here’s a great one.

Written by Jason Lancaster