Fighting a P1121 Code (Gen 2 Prius)

I do not "check for system clogs". Instead, I see what the system does.
Turn the heater off when I park for the night.

Next morning, I turn the car "ignition on" (power button x2 - foot NOT on the brake). Hook up my scantool (one that has enhanced Prius specific ECM data). I look at ECT (engine coolant) and CHS (coolant heat storage) temperature sensors. Both should be near ambient.

Now power on to "ready". The CHS pump should run for around 5 seconds. CHS (aka water outlet) and ECT (aka coolant) should both go up around 100°F then drop some. After that then the engine starts up.
Here's a graph of my 2006 on a cold morning. (It goes from left to right)

All that said, I have yet to see P1121 that wasn't a glitch with the position sensor in the 3-way water valve (but it is possible). The only way I could "see" the glitch is to use an oscilloscope.

I have also seen problems where the hoses to the valve were swapped around.



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There are several error codes for the coolant storage thermos system. The most common problems are a faulty three-way valve, transfer pump, temperature sensors, and clogged system.

https://share.qclt.com/%E4%B8%B0%E7%94%B0%E6%99%AE%E7%91%9E%E6%96%AF%E5%8E%9F%E5%8E%82%E8%8B%B1%E6%96%87%E6%89%8B%E5%86%8Cpdf%E6%A0%BC%E5%BC%8F/repair%20manual/04pruisr/05/2054m/cip1115.pdf

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So then when there is not a call for heat from the dashboard MFD steering wheel whatever is our coolant control valve acting like the water control valve on an old heating system in the '70s basically? It lets the hot water recirculate and go back to the system and through the radiator and do its thing but not through the valve so much or as fast because no one's calling for it? I'm still having a problem with the '09 is why I'm asking and I've changed the coolant control valve from a vehicle that was running had great heat and air conditioning and everything It was put down for body damage in the back and my heat will come and go at random just because the coolant control valve all three ports are on fire they're hot The thermostat that was in that I took out was original apparently it was working The new would I put in seems to be working. It is a Robert Shaw I think I've used their balance stat for 25 years and never had a problem maybe now's the time.

 

The three-way valve can let heat into the thermos during operation when the engine is very hot. In this case, the ECM considers it better to keep the heat in the thermos than to turn on the fan and waste it into the surrounding space.

 

So I guess a better question might be since I have this three-way valve that I removed I've opened it up carefully and looked in it everything moves like it should so I'm guessing the motor may be bad and not moving it or it gets stuck Can I position this valve to a permanent position to leave it open so that all three hoses flow the hot water and then put the cover back on and leave it maybe take the motor out so it doesn't activate I'm sure it will throw a code but my point is it would work You could like make a three-way hose barb I can buy it from a pool supply store I think I can even get one rated to this kind of temperature so the three hoses would just plug up to it and the water would go to the three of them?

 

Then I would be three-way no valve

 

The three-way valve can be set to the desired position and the valve control connector can be disconnected. You can completely eliminate the bypass for the thermos by short-circuiting the line, eliminating the valve and the thermos.
Everything will work, the coolant will run along the short line, the thermostat will operate when heated, the fan on the radiator will turn on when it gets very hot.
But the engine check light will be on the instrument panel.
The ECM will detect a malfunction after some time and light the error lamp.

 

Yes sir I gathered that what I was saying is the little motor that's sitting inside the casing if I'm not mistaken I can just lift that out then when it's called for to close and open the valve it will think it's moving or doing something because power's going to it I don't know but it would still be plugged up so the circuit would be complete whatever that is The valve would be stuck at the position I put it in because there's no pump and spiral gear to activate and move anything kind of like the heater motors so that would be that I figured with it plugged up the circuit would be complete things would be hot and working properly and maybe the computer wouldn't detect a problem but it's just a guess I mean I give it a try because my heater three-way valve is not in a bracket it just hangs there between the three hoses doesn't ever move you can slam on the brakes it doesn't do anything so I can easily lift it in and out without playing games with brackets and whatnot so I'll get the other one ready set it so everything is open take the motor out with the cover on drop it in hook it up and see what happens just be one less thing motor to fool with if it can stay open that would be great or partially open or mostly open or so that I have heat and everything else works and.

 

The computer is smart, sooner or later it will detect that the tap is "stuck" and will try to move it, and when it fails it will light a yellow lamp.
European and Japanese Prius Gen2 do not have a thermos, the system is simpler.