Ford Repair: explorer codes, coolant temperature sensor, engine coolant temperature sensor

Question
hello checking my codes i found
code 12 ....unable to achieve upper test limit.
code 13 ....unable to achieve lower test limit.
code 21 ....ECT out of self test range.
can you please tell me what i should do to fix these problems and are they real problems that will affect my trucks performance. by the way i just changed my engine coolant temperature sensor
about a month ago.also what do these codes mean.

Answer
I don't have a service manual listing these codes.  But if the ECT (Engine Coolant Temperature) Sensor is out of range, the sensor, or wiring to the sensor is bad. To test the ETC sensor.  I believe the resistance in the sensor should decrease while the engine warms-up.

I hope this helps,
Dave