Hello, I just replaced my Coolant Control Valve in order to address the Check Engine Light P1121. When I first got the CEL P1121, I scanned these forums and after some time (hours), I was able to figure out what was needed in order to replace this part. So, to save the next DIYer some research time, I will consolidate it into this post complete with, never before seen, photos of the elusive valve in its natural habitat. If you just got this code, you should first make sure your coolant is not low. Some people have reported their P1121 code going away permanently just by bringing the coolant level up to proper levels. For me, the code would come on and off on its own, even when the coolant level was correct. Part Number 16670-21010 Time to replace (including ICE coolant drain and refill) - 4-6 hours (with breaks) Difficulty level - Intermediate (tougher than an oil change) Total Cost - $120 ($80 for part, $40 for 2 gallons of Toyota Super Long Life Coolant) Dealer Replace Cost - $400-$500 Tools needed: Phillips screw driver (for various clips) Flat screw driver (for various clips) Pliers for hose clamps 10mm and 12mm deep sockets for the valve mount Optional tools (makes it easier): Cable operated hose clamp pliers Link Spill Free Radiator Filler Kit ($20-$25 on amazon) Here is a video showing valve inside and outside of Prius. One poster mentioned that clamping the three hoses allows you to skip the draining and refilling of coolant. This might be a good idea if you have recently replaced the ICE coolant, but chances are this part is going to fail around the time that you should be swapping the coolant anyway. Here is his post in case you want to try it. Link. Procedure: 1. Remove the radiator cover and under engine cover. Mark the upper hose connected to the valve. I used some floursecent paint. This is very important because once the valve is out, the two upper hoses will be difficult to differentiate. 2. Drain the ICE coolant. There are many references on this forum on how to do it. This is my favorite post that covers it. Link. I just removed the engine under cover LH (Driver side), then reached over to the thermos (heat storage tank) and twisted the yellow valve until the fluid start pouring. The LH fender is shaped to drain the fluid out of a hole, so no need to remove the fender trim, too. I did not bother opening the radiator or engine coolant drains. Lets pause for some commentary on the valve. The three hoses attached to it are held on by spring clamps. The largest frustration for this job is trying to get the spring clamps off in the very tight space that you are given. If you are comfortable removing (or raising) the inverter, your life just got easy. Removing the inverter will not only give your pliers easy access to those two upper hose clamps, but it will also make removing the valve mounting bolts a piece of cake. But, the mounting bolts are not really that hard to reach with a tiny socket wrench and some deep sockets. Another option for the hose clamps is to buy cable operated hose clamp pliers. I discovered these after I was finished and would have easily paid the $40 for them. It took me a total of about an hour of to get the three clamps off and back on due to the tight space. With the inverter out of the way, or the cable pliers, I bet I could have reduced that to 5 minutes. These two options are optional. I am just an average DIY and I was able to do it without them, but I would have fell in love with (or at least stared longingly at) any woman that walked in with a pair of those cable pliers when I realized my normal wrenches were too big to reach the clamps, and my coolant was already drained out. 3. Remove the power connector from the valve, then move the coolant drain pan under the valve and remove the three hoses attached to the valve. 4. Now remove the two mounting bolts holding the valve. 5. Remove the valve from the engine compartment. I could only do this by slowly wiggling it downward and out from below (safety goggles will prevent coolant and dirt from falling in your eyes during this). The inverter blocks removal from above. 6. Now attach the two bolts on the new valve and reattach the three hoses and the power connector. 7. Refill the coolant per the procedures in any of the coolant posts. This is the most time consuming process (took me 2 to 3 hours). Do not forget about the bleed valve on the right side of the radiator. There is a big yellow sticker pointing to it and it takes a 6mm hex wrench. I used the paper clip method that Patrick Wong describes to make the heat storage pump run for 15 seconds at a time. Hope this helps the next P1121 victim. Attached Pictures: The first picture shows the valve in yellow, the upper hose in green, and the middle hose in blue. The last picture shows the old and new valve close enough to see what I suspect is the failure. In the old valve, you can clearly see the valve is stuck halfway between open and closed.