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

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    Location:
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    2013 Prius
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    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

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    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

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    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
    BiomedO1 likes this.
  4. CR94

    CR94 Senior Member

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    Has the coolant level dropped significantly recently? If so, and if there are no external leaks, that's not good news.
     
    #4 CR94, Feb 2, 2026 at 10:48 PM
  5. BiomedO1

    BiomedO1 Senior Member

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    Sounds like an intermittent water pump; so if it's working when you take it to the mechanic - he may tell you there's nothing wrong. An ON again, OFF again water pump gets reset when you do a shut-down, restart. The real problem is that your not monitoring your engine temperature. That thermostat light doesn't come on until it's 248+F, when your engine is suppose to be running between 180F-210F. This is how people blow their head gaskets.

    Good Luck....
     
    #5 BiomedO1, Feb 2, 2026 at 11:02 PM
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