Intermittent P0128 code ... what to do?

Discussion in 'Gen 4 Prius Care, Maintenance and Troubleshooting' started by BlueCorollaInTN, Feb 14, 2026.

  1. BlueCorollaInTN

    BlueCorollaInTN New Member

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    Hello! I'm a brand new member, but have lurked here and read posts for a long time.

    As you might guess from my username, my car is a 2020 Corolla Hybrid (basically the same powertrain as a 2019 Prius). It has been fantastically dependable, only requiring normal maintenance and wear item replacement.

    Yesterday, it threw a trouble code for the first time ... "P0128" ("thermostat stuck open"). I don't notice anything special in the way of a slow warmup, heat not working, etc. It happens after driving the car for maybe 15 minutes or so, parking, then driving briskly again (e.g., a trip to the store, in other words) -- but it doesn't always happen at all. One trip today tripped the code, another a couple of hours later did not.

    Today I drove around with my scan tool connected and monitoring coolant temperature, and convinced myself that my engine coolant temperature sensor was fine ... showed the engine gradually warming up to 190 F. I also checked with an IR thermometer that the radiator hose next to the temp sensor was hot, but not as hot as the sensor read (e.g., 180 degrees on the sensor, 150 degrees on the hose) ... if it had been hotter than the sensor I would consider that a problem.

    I was planning to do a coolant change anyway, but maybe not until the fall. Should I just go ahead and do the coolant change now, together with a thermostat replacement, or is there some other failure cause I should consider first?
     
    #1 BlueCorollaInTN, Feb 14, 2026
  2. BiomedO1

    BiomedO1 Senior Member

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    P0128 is basically saying that your engine is taking too long to warm-up. That's why it doesn't throw that code all the time. Your thermostat could be stuck partially open so monitoring your engine temperature to 190F is kinda irreverent; since you didn't time it. Don't know if that car has an exhaust gas heat recovery system in place. If it does, that pathway may be blocked and not heating up the system fast enough. It's not recovering engine exhaust heat and your system is taking longer than projected to warm-up. If you remove your thermostat and find it fully closed - I'd be looking at that exhaust gas heat recovery system operations.

    Good Luck...
     
    #2 BiomedO1, Feb 14, 2026
  3. rjparker

    rjparker Tu Humilde Sirviente

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    According to a search, a 2020 Toyota Corolla Hybrid has an Exhaust Heat exchange (EHex) system which helps some of the coolant to quickly warm up the throttle body before the thermostat opens.

    A coolant-temperature sensor downstream of the EHex system will read hotter temperatures before a second coolant temp sensor monitoring the engine and the large return radiator hose warms up.

    If the thermostat is stuck open when the engine is cold, full flow into the radiator will slow engine warmup. Normally it takes 5 minutes or less for the engine to warm up since most flow is initially blocked by the thermostat.

    You probably have a bad thermostat.

    The diagram below is a similar dual passage cooling system of a 4th gen Prius. Remember flow into the thermostat's bypass inlet is not blocked by the thermostat but radiator flow will be when cold.
    Gen4 coolant flow w notes.jpeg

    Techstream and most scanners label both sensors "coolant temp/temperature." Each is read by a separate ecu; typically the Engine ecu for the Head mounted sensor and the Combination Meter ecu for the Exhaust Heat exhanger sensor.
    Engine and ECT Coolant Temp.jpeg

    Combination Meter Coolant Temp.jpeg

     
    #3 rjparker, Feb 14, 2026
    Last edited: Feb 14, 2026
  4. EchoEcho

    EchoEcho Junior Member

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    Anecdotal addition... Recently changed thermostat on a 2017 Prius (135000 km) due to a slow gasket leak. After the change noted faster warmup and more consistent coolant temp in high 80s Celsius.
     
    #4 EchoEcho, Feb 20, 2026 at 7:40 PM
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