heat stops after 10-15 mins - AC room temperature sensor?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by dex3703, Dec 1, 2025 at 6:30 PM.

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  1. dex3703

    dex3703 Member

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    My 2005 Prius with 173k miles has a problem with the heater deciding to not blow heat after 10-15 minutes of operation. I've had the car since 2013, and the heater/AC has worked fine until the last year or so. AC seems to work fine--I just keep it at max cold--but the heat has started cutting out after 10-15 minutes in the winter. Pleasant 65 or 66 becomes ice cold. I can sometimes get it to return by cranking the temp above 70.

    I'm guessing it's related to the AC cabin temperature sensor, but wanted to ask before digging around. I've seen this mentioned in these threads, though these threads don't speak specifically to my issue.

    AC issue | PriusChat
    Any idea what this is? | PriusChat

    Thanks in advance.
     
    #1 dex3703, Dec 1, 2025 at 6:30 PM
  2. ChapmanF

    ChapmanF Senior Member

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    It's a thermistor, not something you often hear about going bad. But a simple way to check would be to watch the room temperature sensor live data on a scan tool and see if it is or isn't showing a believable reading, before and/or after the heat stops coming.
     
    #2 ChapmanF, Dec 1, 2025 at 6:47 PM
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  3. PriusCamper

    PriusCamper Senior Member

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    That's the amount of time it takes for the heating system to transition from electric over to traditional engine heat via the coolant. So the system that does that would be a good place to start looking. Watching the the live data like @ChapmanF says will help... Also making sure you don't have any air in the coolant system and ensuring its running at a normal temperature needs to be checked too.

    I think in reality though you don't have any problems with your car, just that its been down to freezing in Seattle area past couple days and setting your cabin temperature at 65 isn't going to warm up the car very well and you need to turn the heater all the way up to hi... As in a vehicle thermostat doesn't work like a house thermostat. For best results get the car as warm as possible as quickly as possible and control the vents and fan speed to optimize cabin temperature.
     
    #3 PriusCamper, Dec 1, 2025 at 7:02 PM
  4. ChapmanF

    ChapmanF Senior Member

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    If this is a gen 2, though, I don't think it has any more electric heat capacity than gen 3, like somewhere around 700 watts.

    That has never been enough to make a difference I can feel. It does make a noticeable difference in how fast the windshield clears, but the air still feels cool to my hand. The 700-ish watt electric element is just nowhere near close to the rated 5300 watts of heat that can be extracted from the coolant once the engine's warmed up.
     
    #4 ChapmanF, Dec 1, 2025 at 9:39 PM
  5. Brian1954

    Brian1954 Senior Member

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    The first thing to do is check the level of the coolant in the radiator. When the car is cool, remove the radiator cap and make sure the coolant is all the way to the very top. The next thing is to check the level of the coolant in the plastic overflow bottle.
     
    #5 Brian1954, Dec 1, 2025 at 9:54 PM
  6. dex3703

    dex3703 Member

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    Well whaddyaknow--the coolant is a little low. I've never seen that in this car. I had the coolant-inverter coolant changed at the dealership a couple years ago. Guess I'll add some and see if that helps. No warnings or other indicators--no sign of leaks.

    Is it the same coolant for the radiator and the inverter? The bottle next to the inverter could also use a little, though it's not to the 'low' line.
     
    #6 dex3703, Dec 1, 2025 at 10:10 PM
  7. Brian1954

    Brian1954 Senior Member

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    Yes, both cooling systems use the same coolant.

    After filling the radiator, come back and tell us if that fixed the problem.
     
    #7 Brian1954, Dec 1, 2025 at 10:13 PM
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