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

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    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

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    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
    Norry, Air_Boss, Brian1954 and 1 other person like this.
  3. 2015V-BadHG

    2015V-BadHG New Member

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    RJ, tks for this great explanation of coolant flow.
    Pete
     
    #3 2015V-BadHG, Dec 30, 2025 at 12:46 PM
  4. Norry

    Norry Junior Member

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    1. As in I flushed it with a hose. Water came out the other side and reversed it too. Did not see anything out of the ordinary coming out apart from the clear water once the coolant had been flushed.

    2. To change this it would still mean removal of the whole dashboard. There is a video on YouTube showing how the blend door actuator can be replaced (assuming it's the same part). How would I know this is actually bad?

    3. Recently drained and refilled, problem remains as is. Any way to tell if there is gas in the coolant?

    4. Are these the hoses that connect to the thermostats? If they are, then when replacing the thermostat a lot of the coolant did drain out. I stuck a new thermostat on and squeezed the hoses when trying to get the air out of the coolant. Is there any way to check if this is bad?

    Your explanation of how the coolant works is incredible. Do you think there could be an issue with the cooler? I did replace the egr valve and had the cooler cleaned back in 2022. When removing the intake manifold and egr pipe, it wasn't too clogged.



     
    #4 Norry, Jan 1, 2026 at 1:54 PM
  5. bisco

    bisco cookie crumbler

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    idk, but tech stream might be able to identify a bad door
     
    #5 bisco, Jan 1, 2026 at 2:03 PM
  6. 2015V-BadHG

    2015V-BadHG New Member

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    Norry, I bought my 2015 V with 251K miles on it with a failing HG. The heat was OK, but occasionally there was no heat like your car.
    What happens is blow by from a seeping HG pressurizes the coolant system and forces coolant out of the over flow tank. As well as creates air in the system. What I did to limp my V home (Salt Lake City to Portsmouth NH 2500 miles!) was to remove the rubber Oring from the over flow tank to keep system from pressurizing and create a makeshift over flow tank for the overflow low tank so
    I could recover coolant and fill the overflow tank back up. LOL. The key is to get the air to burp out of the system at high RPM while driving.
    Then "hope" you can get enough coolant back into via a overfilled coolant tank while car cools off overnite. I did manage it, got heat to work and ran it on high over maintains to keep car from overheating.
    Bottom Line is you need a new HG!

    Pete
     
    #6 2015V-BadHG, Jan 1, 2026 at 2:10 PM
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