ECT Below Thermostat Regulating Temperature code 128. How do I fix the problem

Is your temperature gauge on the dash reading lower than normal? If so, suspect the thermostat. With the engine warmed up, the radiator and heater hoses should be too hot to hold onto for very long. That code means the engine isn't getting hot enough. That can lead to excessive engine wear over time since the pistons are designed to expand more in one direction when warm so they fit properly. Also, fuel won't vaporize as well when the engine is too cold so it won't burn efficiently. If you do have to replace the thermostat, be sure to burp the air bubbles out before warming up the engine. Thermostats don't open in response to hot air. They have to be hit with hot liquid. Air pockets will lead to overheating when the thermostat doesn't open.

If the engine seems to be hot enough, you'll need a scanner that displays live data to see exactly what the computer is seeing for temperature. It is extremely rare for temperature sensors to fail, but it is possible.