Hyundai Repair: PO118, coolant temperature sensor, coolant leaks

Question
I have a 2001 Hyundai Sonata V6. The check engine light has been coming on and then oddly going off. I took it to Auto Zone and they said it was PO118 Engine Coolant Temperature Circuit High Input. I would like to know how to replace this sensor and test the sensor if possible.  

Answer
A scan tool will be the best way to diagnose this problem.  We already know that the problem is intermittent because your check engine lamp will go back out on its own.

When a diagnostic trouble code sets, your engine control module (ECM) stores what is called freeze frame data.  Basically, these data are the readings from several sensors and computer outputs when the trouble code sets.  You'll be primarily interested in the coolant temperature when the code set.

If the temperature was -40 degrees, then there was an open circuit at the time the code set.  

A high temperature could indicate that the vehicle was overheating or that there was a short circuit.

Assuming you don't have access to a scan tool, here's what I'd recommend:
-- Check your coolant.  If your coolant is low, check for and repair any coolant leaks.  
-- Once you've verified the coolant is at the proper level, remove the radiator cap and start the engine.  Check to make sure the coolant flows and that the radiator fan cycles on and off without the coolant boiling.  It will take up to a half hour for your engine to reach operating temperature.  The purpose of this is to verify that the cooling system is working properly and that the lamp wasn't coming on because the engine was actually too hot.
-- If the coolant level is okay and the cooling system is functioning normally, replace the sensor.  This isn't 100% because we haven't checked the freeze frame data to see what type of issue we have, nor have we checked the wiring between the sensor and the ECM.  But it is the case that the vast majority of incidents involving P0118 are due to a faulty coolant temperature sensor.

The coolnat temperature sensor is located in front of the coolant housing on the transmission end of the engine.  If I recall correctly, it's under the ignition coil.  To replace it, unscrew the old one and screw in the new one.  Then top off the cooling system to replace any coolant that came out during servicing.