I have read a lot, but would love a distillation on P1121

Yes, my 2007 just turned over 300K miles and I got a P1121 code. I am not low on coolant and my water pump does not appear to be leaking. I believe I need to replace the valve. I see lots of options for replacement, but am leaning towards one in a Toyota box. I see them on eBay for around $80, but are they real? I see 3rd party brands on Rock Auto for about the same ($75-$80). Should I just go to the dealer and buy one or aftermarket part is ok for this? I do have some coolant on hand and I do plan to pinch off hoses to reduce the loss of coolant, but I guess I will struggle to bleed it.

Thank you in advance and I apologize for not reading through gigabytes of content on this subject. I did get through a lot and then realized it's bigger than me.

Cheers and happy summer solstice!

 

Thank you for the responses. I realize there is more than one reason for the P1121 code, but it seems that it's almost always the valve. I ordered the valve from my local(ish) dealer with online discount and it was around $120 with local pick-up. They did not have it in stock and I should get it in a day or two. We'll see how this saga plays out... I hope it's just the valve and nothing more complicated. I will update this thread as I make progress. In any case, 300,000 miles and rocking. I love this car. Want another gen 2.

 

I received the valve yesterday Genuine Toyota part from the dealer at just under $120 after taxes) and replaced the old one with it. It all worked pretty well. Not a fun job. I took the left headlight out. I think it's the best way. I also had to take some fasteners out from the bottom plastic shield and many of them disintegrated. What do people do to replace those? The P1121 code has not come back and the coolant temperature is staying below 195F (mostly hanging out at 186F). I followed a simple bleeding process of opening the bleeder valve on the radiator and adding coolant to the radiator filler orifice. I also made sure my expansion tank was full and I ran the pump feeding the thermos to help take up the coolant. I drove the car with heat set to hi and the OBD2 dongle reporting coolant temp in real time and made sure the car did not overheat. At this point I just need to replace those broken plastic fasteners on the underside.