Thermostat & cooling question

My "check engine" light came on recently and I took my vehicle to a local auto store to check the codes. They found P0126 & P0128. I looked up the codes and found probable causes being low coolant level, open or short circuit condition or faulty ECT sensor. I checked the coolant level and saw that it was right at the minimum line. I added some coolant and waited a few days to see if that did anything. Nope, the engine light is still on. I've been scouring the internet for any help and keep seeing that the problem could be that my thermostat is stuck open and should be replaced. I'm a DIY kind of girl and if I have the tools to fix it, I can. Plus, I saw what people were paying and I don't think I could afford that right now. So, I guess I'm looking for any help or guidance in fixing this situation. In my state, we have to pass emissions testing before renewing our tags and I have until January to do so. Any comments, help, suggestions, etc would be greatly appreciated!