Red Thermometer Light and Heater Not working Intermittently.

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by srdamodaran, Feb 2, 2026 at 3:46 PM.

  1. srdamodaran

    srdamodaran New Member

    Joined:
    Jun 21, 2025
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    Location:
    Fairfax, VA
    Vehicle:
    2013 Prius
    Model:
    Three
    Have a 2013 Prius with almost 275000 miles.

    Two times in the past two weeks, the red thermometer light came on for a few seconds. I stopped the car, restarted it, and the light went off...and stayed off.

    Over the past month, the heater doesn't turn on intermittently while driving (even after a few minutes). Again, I restart the car and everything works fine.

    I don't have any coolant leak, though I'm almost at the point where it needs to be changed.

    I haven't changed the water pump or the thermostat ever.

    Could it be just a problem sensor on the dashboard or something else?

    I will get it checked out by a mechanic, but I want to get some ideas on what it could be before I go.

    Thanks.
    Raj.
     
    #1 srdamodaran, Feb 2, 2026 at 3:46 PM
  2. ASRDogman

    ASRDogman Senior Member

    Joined:
    May 29, 2018
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    Location:
    Florida
    Vehicle:
    2010 Prius
    Model:
    Two
    Have you replaced the head gasket yet?
    It is possible that the impeller is starting to crack and not spinning fast enough.
    You should test it with a diagnoses tool to verify the speed is what it's suppose to be.

    Also, if the thermostat is sticking, it could be failing.

    Have you replaced the coolant at the 100,000 service and 50,000 miles afterwards?
     
    #2 ASRDogman, Feb 2, 2026 at 4:28 PM
  3. Brian1954

    Brian1954 Senior Member

    Joined:
    Jan 31, 2021
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    Location:
    South Central PA, USA
    Vehicle:
    2011 Prius
    Model:
    III
    It sounds like the engine water pump may be going bad. Use a scanner to monitor the engine coolant temperature or the pump speed while you drive the car.
     
    #3 Brian1954, Feb 2, 2026 at 5:29 PM
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