Idea: Low Coolant Sensor to Prevent Head Gasket Failure on Gen 4 Prius

Discussion in 'Gen 4 Prius Care, Maintenance and Troubleshooting' started by PatrickL, Sep 19, 2025 at 8:54 PM.

  1. PatrickL

    PatrickL Junior Member

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    I wanted to float an idea here because I think it addresses a serious problem that a lot of Gen 4 Prius owners are facing.

    As many of you know, when the coolant level drops, the check engine light doesn’t come on until you’re practically empty -- and by then, it’s too late. Within minutes you risk blowing a head gasket. (This is what happened to me). It seems unbelievable that Toyota hasn’t already solved this, but the fact is, there’s no early warning for low coolant.

    My idea is simple: an aftermarket low-coolant level sensor that sits in the reservoir and alerts the driver before things get critical -- say, as soon as the level dips below the midpoint between “Full” and “Low.” A float sensor or probe could trigger a light or buzzer in the cabin.

    The way I see it, this kind of kit could retail for maybe $100–$200. That’s pocket change compared to the $3,000–$5,000 cost of replacing a head gasket or engine. And there are millions of Gen 4 owners out there who might want the peace of mind.

    So here’s my question:

    Would you be interested in a product like this?

    Do you think it’s worth pursuing as a DIY aftermarket kit?

    Considerations:

    Market potential

    Target audience:     Owners of Prius Gen 4 (millions of units worldwide) plus other Toyotas with similar cooling layouts.

    Aftermarket appeal:     This could be an add-on kit sold through auto parts chains, Amazon, or Prius enthusiast forums.

    OEM potential:     Toyota or other automakers might license or adopt it if shown to reduce warranty claims and engine failures.

    Challenges

    Sensor reliability:     Coolant can be corrosive, and sensors must handle heat and vibration.\

    Calibration:     It must avoid false alarms (like when coolant sloshes around).

    CARB/EPA considerations:     If marketed in California, aftermarket add-ons sometimes need regulatory clearance (though a warning sensor is much easier than an emissions-control device).

    That's all I can think of for now...

    I’d love to hear what the PriusChat community thinks. If there’s real interest, I might take the next step and see about getting a prototype made.

    Thanks,
     
    #1 PatrickL, Sep 19, 2025 at 8:54 PM
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  2. BiomedO1

    BiomedO1 Senior Member

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    don't take this the wrong way; but why reinvent the wheel??? There are already existing sensors in the car and all you have to do is monitor the engine coolant temperature.

    If your cheap, why not pop the hood and check your engine oil level and eyeball your fluid levels while your there? I was thought to do that at every fill-up. I'm not one to stop someone from spending money that's burning a hole in their pockets - just so they can avoid popping the hood and doing some basic checks that your owners manual tells you should do.......

    You do You.....
     
    #2 BiomedO1, Sep 20, 2025 at 1:16 AM
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  3. Mendel Leisk

    Mendel Leisk MMX GEN III

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    Gen 4 coolant dropping is usually due to a defect in the exhaust heat recovery system, defective welds or similar, allowing coolant to leak into the exhaust. Adding a low coolant sensor seems a poor solution, to put it mildly.
     
    #3 Mendel Leisk, Sep 20, 2025 at 9:51 AM
  4. Doug McC

    Doug McC Senior Member

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    I agree. Spend money to install a device that could fail (considering the environment it has to operate in) instead of visually checking the coolant level? OKAY, if that’s what you want to do. You’ll still need to pop the hood to check the oil level and brake fluid (unless you want to add more devices that increase the likelihood of failure).
     
    #4 Doug McC, Sep 20, 2025 at 10:53 AM
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  5. Mendel Leisk

    Mendel Leisk MMX GEN III

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    On the other hand, if a low coolant sensor was designed and implemented at the factory, it could be a good safety-net.
     
    #5 Mendel Leisk, Sep 20, 2025 at 11:32 AM
  6. BiomedO1

    BiomedO1 Senior Member

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    IMHO; They shouldn't have removed the coolant temperature gauge from the instrument cluster. Add to that the ECU doesn't even log an engine over temp incident. That simple adjustment to the software would allow Toyota to say - "look at how may times you ran it hot." and your surprised the engine or head gasket blew?????

    Getting off the soap-box now......
     
    #6 BiomedO1, Sep 20, 2025 at 12:12 PM
  7. BiomedO1

    BiomedO1 Senior Member

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    I think they actually want to be driving a BMW or MBZ; no dip sticks. Just a $ icon in the dashboard which indicated "take me to the dealership" and I hope your credit card isn't maxed out.o_O:D:LOL::ROFLMAO::eek::whistle:
     
    #7 BiomedO1, Sep 20, 2025 at 12:35 PM
  8. bisco

    bisco cookie crumbler

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    i think it's brilliant. the coolant light comes on after the head gasket blows.
    if i had an early gen4 with the original heat exchanger, i would absolutely buy one.
     
    #8 bisco, Sep 20, 2025 at 1:46 PM
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