Where is the coolant temperature sensor located and how to replace it?

What was the actual code? It wasn't P0128 by chance? Asking because I'd hate to see you swap parts that are not the correct items. The P0128 for instance is 99.999% a bad thermostat. That temperature sensor, however, is hiding behind the alternator in the intake manifold. Remove the air cleaner and look behind the alternator, unplug the connector, then prep the new sensor. Now remove the old sensor, just before you remove it, push down a bit and squeeze the upper radiator hose. Now let the hose go, pull the old sensor and insert the new one fast. You should be able to do it and only leak a tablespoon of coolant. Gently tighten the sensor to 60 Inch pounds. Be careful as it's possible to damage the manifold and cause leaks.