Check engine light keeps going on and off, what to do?

In the past couple weeks the check engine light on my 07 Prius has gone on 3 times. The first time it went off I adjusted the gas cap and it went off the next day. It went on again this Thursday, but then turned off today when I drove to the store. Later on in the afternoon though, it turned back on when I made another errand trip. I adjusted the gas cap a couple days ago, but in between the two errands I did today I adjusted nothing.

The car drives totally fine and I'm able to start it without issue. Two things I've noticed though. First, the two times I've gotten gas this weekend, I've only been able to fill about 4-5 gals before the pump stops and won't let me fill any more. This is after the "add fuel" message comes up with one blinking bar on the gas gauge. When I fill it, the fuel gauge still goes full, even though I used to be able to put 6-7 gallons into an empty tank.

Second. Earlier in the day on Thursday, before the light went on, my key fob stopped working. I'm still able to lock the car with the button on the door but the fob still doesn't work even though I changed the battery.

Any ideas?

 

I'll try that text at auto zone. Oil level is good, I had it changed recently.

 

Went to autozone and the error code was "coolant flow control valve position sensor stuck" and suggested a new heater control valve. I opened up the hood and saw the engine coolant was fine, but the inverter coolant was low. Can I top it off with Toyota Super Long Life Coolant and be good or do I have to bleed and replace? Prius is at about 115k miles.

 

The check engine light went off again after I had the error code read at Autozone, though I still went and filled the coolant. I'm going to wait and see if the light comes on again. If it does, and the coolant level is fine, I will probably take it in. I found a post on Priuschat about how to replace the heater control valve but I have little to no experience working with the car so I will likely take it to a mechanic unless you guys think it's an easy enough fix. That post also said that sometimes when that check engine code (P1121) goes on, it simply means the coolant level is low. So I'm not sure if I solved the root of the problem or not.