Heat stopped working: 2012 Two, 145K Miles

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by indel, Feb 15, 2026 at 8:27 PM.

  1. indel

    indel Member

    Joined:
    Apr 1, 2024
    251
    78
    1
    Location:
    Northwest Indiana
    Vehicle:
    2012 Prius
    Model:
    Two
    The heat has been working fine all winter. In fact it's so strong that I have to keep it around 68F to not get too hot. We are having warmer weather this week in the midwest. On a long drive this evening, the heat won't work properly. I was blasting it at 75F and still cool air.

    - 2012 Two at 145K miles
    - Both coolant tank levels are at the top mark and have been consistently there for the 3000 miles I have owned the vehicle
    - ICE coolant temp hovers around 88-91C (190-195F) - see below
    - No CEL or heat malfunction indicator

    How should I go about diagnosing this problem?

    IMG_8434.PNG
     
    #1 indel, Feb 15, 2026 at 8:27 PM
  2. rjparker

    rjparker Tu Humilde Sirviente

    Joined:
    Jun 6, 2008
    10,375
    6,495
    7
    Location:
    Texas Hill Country
    Vehicle:
    2012 Prius v wagon
    Model:
    Three
    1. Read second coolant temperature sensor located downstream of the heater and exhaust heat recirculation system. This is a dual passage cooling system.

    If there is air in the heater core the second sensor will be low.

    Car Scanner app sensors IMG_0556.jpeg

    Techstream Sensors
    Combination Meter Coolant Temp.jpeg Engine and ECT Coolant Temp.jpeg

    2. Verify heater blend door is moving. One method is to turn blower on high and switch temperature from Lo to High and back. Air flow will slow as blend door allows air through heater core.

    Obviously there should be air flow.

    3. Verify water pump flow in Coolant Reservoir when coolant has opened thermostat. Use flashlight to observe flow from top reservoir hose.
    IMG_0557.jpeg
     
    #2 rjparker, Feb 15, 2026 at 10:38 PM
    indel likes this.
  3. indel

    indel Member

    Joined:
    Apr 1, 2024
    251
    78
    1
    Location:
    Northwest Indiana
    Vehicle:
    2012 Prius
    Model:
    Two
    Very helpful, thank you! I will check these out tomorrow.
     
    #3 indel, Feb 15, 2026 at 10:59 PM
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