No Heat - Replaced multiple components

Discussion in 'Prius v Care, Maintenance and Troubleshooting' started by Norry, Dec 29, 2025 at 3:01 PM.

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  1. Norry

    Norry Junior Member

    Joined:
    Feb 14, 2023
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    Location:
    United Kingdom
    Vehicle:
    2010 Prius
    Model:
    III
    I have a 3rd Gen Prius which has done 300k+ miles. The car has recently started to give a few issues.

    The main issue is that I currently have very limited/no heat coming through the vents.

    I have checked and replaced the following:

    1. Tried to flush heater core with hose pipe. No change.
    2. Replaced water pump. No change.
    3. Replaced thermostat. No change.
    4. Ended up cleaning intake manifold and throttle (probably has no effect on heat but I had it open so why not!)

    The last thing I can think of is the heater core itself. There used to be a step by step video on YouTube showing how to take off the dashboard etc. Can't seem to find it now. Anyhow this is the last resort.

    Does anyone know what else it could be apart from the heater core?

    Could I use a coolant flush chemical like liquid moly to perhaps agitated any debris within the heater core thereby returning heat?

    At a crossroads and not sure what to do.

    Note:
    If anyone ever cleans their intake manifold and gets either a P0121 or P0443 code, then check all the clips and hoses are connected to the throttle body. Had to remove the air intake assembly twice because I kept on missing things.
     
    #1 Norry, Dec 29, 2025 at 3:01 PM
  2. rjparker

    rjparker Tu Humilde Sirviente

    Joined:
    Jun 6, 2008
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    Location:
    Texas Hill Country
    Vehicle:
    2012 Prius v wagon
    Model:
    Three
    .

    1. "Tried" to flush? Does this mean you could not get flow through it?

    2. The "air mix control damper" blend door in the air handler may not be opening. Some have found the mechanical link disconnected.
    IMG_0063.jpeg

    3. There could be air in the heater core typically from a head-gasket leak pushing combustion gas into the coolant. This problem often temporarily goes away with a drain and fill.

    4. The thermostat-housing bypass inlet could be clogged.

    Heater Coolant Flow

    Heater coolant flow is continuous and has no valves in its path. The "air mix" blend door opens to allow interior cabin flow through the heater core.

    The engine's big hose coolant flow is separate from the heater-> egr cooler/exhaust recirc -> thermostat bypass flow.

    The heater coolant flow enters the core through the bottom hose and exits through the top.

    After leaving the heater core it takes two paths, one through the egr cooler and a second parallel path through the exhaust heat recirculation system.

    Those two flows merge at the front of the egr cooler and return to the small thermostat bypass inlet which is designed to be open all the time.

    Egr cooler w flows pic.jpeg

    From the thermostat it goes to the water pump and then through the engine.

    From the engine it returns to the heater core's bottom hose.
     

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    #2 rjparker, Dec 30, 2025 at 11:47 AM
    Last edited: Dec 30, 2025 at 12:27 PM
    Brian1954 and 2015V-BadHG like this.
  3. 2015V-BadHG

    2015V-BadHG New Member

    Joined:
    May 19, 2025
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    Location:
    Portsmouth NH
    Vehicle:
    2015 Prius v wagon
    Model:
    Three
    RJ, tks for this great explanation of coolant flow.
    Pete
     
    #3 2015V-BadHG, Dec 30, 2025 at 12:46 PM
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