Coolant reservoir cap was open, 2 weeks later code P1121

2006 Prius, ~165,000 miles

Almost a year ago I got my coolant replaced. 2 weeks ago I noticed the engine coolant reservoir cap was off, and the coolant was low. I refilled the reservoir, car doesn't appear to have lost coolant since. A few days ago the CEL came on (code P1121). My understanding is that P1121 usually is just bad for gas mileage, but my concern is that the open cap on the coolant reservoir two weeks ago might indicate a deeper issue that I'm missing.

Timeline:

• May 2023: Mechanic flushed + replaced my coolant @158,738 miles. I do not know which coolant they used, but it's red.
• Jan 27: Noticed the engine coolant reservoir cap was open, and that the coolant was right at the "low" line. Thinking back now, I realize my car was on an incline, so it was actually below the line. I closed it, drove home (~1 hr on the interstate), did not touch it again until it was time to refill the coolant.
  • Jan 28: I bought NAPA red "Asian" coolant
  • Jan 29: I drove it to the nearest level outdoor parking space (~1 small city block) and filled both coolant reservoirs to the "full" line. At this point the engine reservoir appeared empty.
  • drove quite a lot in following week due to family member in hospital, both on interstate and within a city
• Feb 8: CEL came on with code P1121 (I have an OBD2) while driving home. The readout description seemed like just a dirty throttle, so I finished driving home (~30 minutes through city traffic).
  • 2.5 hours after getting home + turning my engine off, I checked the reservoir. Compensating for incline, the engine coolant reservoir appeared to still be at the full mark.
  • Feb 9: checked under the radiator cap last night. Coolant was actually slightly too high.
  • This morning (Feb 10) I drove the car again, to the pharmacy up the street. Drove fine, did sound slightly rough on startup. CEL has not gone off since coming on

So:

1) Am I correct that the typical cause of a P1121 error (a failure of the 3-way valve) does not negatively impact essential functions of the car or lead to engine damage?

2) Are there other plausible causes for a P1121 code? Given what I said about the coolant reservoir two weeks ago, are any likely? I was thinking it could be that I got an air bubble in there at some point, and that's what's gummed up my valve.


My car is the only one my recovering family member can easily enter + exit, and with the hospital bills I would very much prefer not to throw money at a frivolous repair right now. Thank you to anyone who replies, I very much appreciate it.

 

Yeah, I've been keeping an eye on it. Haven't caught a leak yet, but it's possible that it's just too slow to be noticeable this soon.

Didn't occur to me the cap could just be loose. I was thinking either some mechanic messed up or something internal created a pressure buildup. But if that were the case I think I'd be seeing other issues flare up. Thank you.

Do you know of a way to "silence" the P1121 code so that I won't miss another code popping up?