Bad ECT sensor wiring or thermostat?

My check engine light is on. I looked it up online and seen that three people had the same problem with their cars. The temperature gauge is at 0. I know that could be the thermostat stuck open. I was thinking it was electrical like these other people and they replaced the ECT sensor and worked fine so I did that first replaced the ECT sensor. I worked a little while and went back engine light and temperature gauge is at 0. I took it to AutoZone have to read the code. It was a P0128. The temperature gauge has been coming off and then on. Few days it would go back to normal and suddenly it would drop to 0 and the engine light come on. I have been driving and it would be normal and suddenly the temperature gauge would drop to 0 like a rock not slowly go to 0 just to me like it lost power. Just wondering if there is a way to figure out what is going on. I just don't want to replace the thermostat too and it comes back again. I wish I had a better reader than what AutoZone has but don't. Any help would be greatly appreciated.