P0128 fan is constantly on when dash says — — —, P0014 sometimes comes on

Code p0128 is constantly on and p0014 comes on occasionally. It’s about 7 degrees Fahrenheit outside today and my fan near the front bumper is on and thermostat says 95 degrees Fahrenheit. My car has been warming up for twenty five minutes now. I cleared the p0128. The fan stop, heat got hot again, and now the temperature is going up. I replaced the thermostat already with a fail safe one because of this issue and I’m still having this problem. What’s the next step to replace: cooling fan, coolant temperature (ECT) sensor, or intake air temperature (IAT) sensor?
The fan kicks on when the coolant goes - - - on dash. Coolant was replaced with thermostat. Coolant level is exactly where it should be.

For p0014 oil has been replaced but I pretty sure he didn’t replace the oil filter. I know 02 sensor was replaced. Car rpm's drop then it will throw the code. Also, if it acts sluggish for too long it will throw the code. Should I start with oil filter, actuator, or a sensor? Mechanic says to start with cam or crankshaft sensor, but when looking it up it replacing the sensor doesn’t make sense.