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Low on gas and then P1121 coming on and off for 400+ miles

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Lauren Everett, Jun 21, 2022.

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  1. Lauren Everett

    Lauren Everett New Member

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    Hi everyone,

    I've searched and searched for an answer to this but no luck, so hopefully someone here has experienced this or has some idea of what's happening.

    So first of all, I promise I will never drive with low gas again! I have done it many times in the past - with both this car and others - and have never run out of gas or had any issues that I'm aware of. However, about a month ago I let it get pretty low. The check engine light came on and I immediately headed to a gas station to add a few gallons (the nearest one was very expensive) and then added more when I was closer to home and at my regular station.

    The light was on for a few more starts and then went off right before I tried to read the code with the Auto Zone reader. However, it came back on some miles later and I brought it in and got P1121: Coolant Flow Control Valve Position Sensor Circuit Stuck. Granted, I know very little about cars, but it still seems puzzling that this could be related to the gas...

    Anyway it went off, came on again a few hundred miles later, I took it in for a read and same thing. Went off, and then about 20 miles later went on again. What the heck is going on??? Do I actually need to replace this part, or does the sensor just need to be cleaned, or is it possibly something else? Super bummed because I've had this car for a year and a half without issue until now :(
     
    #1 Lauren Everett, Jun 21, 2022
  2. JC91006

    JC91006 Senior Member

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    It has nothing to do with running low on gas, this p1121 is a valve issue. You need to replace the 3 way coolant valve. They go bad over time.

    But I'm surprised it would come and go, maybe you should check your coolant level and make sure you have enough coolant in the car. Open the radiator cap and check (not look at the reservoir)
     
    #2 JC91006, Jun 21, 2022
  3. PriusCamper

    PriusCamper Senior Member

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    HI Lauren, I live a couple hours North of you and this is a common problem in a Prius... Specifically the sensor wears out. But the good news is the part can be had for $50 and repair takes a few hours: https://www.ebay.com/itm/394097566363

    And even better news is that you can procrastinate on this one for as long as you need to without damaging your car, just lower MPG. I've known people who go for years with this problem, but I don't recommend it because you won't know if your check engine light has come on for other problems.
     
    #3 PriusCamper, Jun 21, 2022
  4. pasadena_commut

    pasadena_commut Senior Member

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    When mine went the coolant level was fine and the light would go on and off.

    The OP should look for the thread on how to replace that valve. It is kind of a PITA and best to read up first. Also be really careful taking off the nut on the topmost/vertical post. On my car and at least one other that nut would not come off and it eventually snapped the stud. (It would turn, but not rise upwards.) In my car it left enough of a nub that the replacement valve's flange could hook over it and it would not come off once the other (horizontal) nut was firmly attached. I'm not sure why this happens. Maybe the nut is much harder metal than the stud, and it strips the threads on the rod and then cuts through it entirely? Either that or somebody at the factory puts these on too hard with a power tool, and it strips the rod at that point, where it lies in wait to ambush an owner later.
     
    #4 pasadena_commut, Jun 21, 2022
  5. Lauren Everett

    Lauren Everett New Member

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    Thanks everyone! I'm just now seeing these, and the light has gone off and on once more since I posted. I guess ultimately I just have to address the problem and get over the weirdness of how this seemed directly related to the gas being super low, but does anyone have any thoughts on that? Total coincidence?

    The other weird thing is that I'm able to use both AC and the heater when the light is on, which from what I understand shouldn't be possible when this part isn't working. My concern is that the code is somehow being triggered by something else. Is that possible??
     
    #5 Lauren Everett, Jul 8, 2022
  6. bisco

    bisco cookie crumbler

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    you might need tech stream to find sub codes
     
    #6 bisco, Jul 8, 2022
  7. mr_guy_mann

    mr_guy_mann Senior Member

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    Given how often these valves go bad, it's over 90% that your code is caused by the valve itself. I pretty much go straight to replacement as a diagnostic step. I have yet to see one that wasn't the valve. (Only use a Toyota replacement from a reputable source)

    You would need some special equipment (a GOOD scantool and a labscope) to test and prove that the valve is at fault.

    There are several different ways that the valve can glitch and trigger the same P1121 code, but not every failure mode will affect coolant flow to the heater.

    Posted via the PriusChat mobile app.
     
    #7 mr_guy_mann, Jul 8, 2022
    Last edited: Jul 8, 2022
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