My sister drives a 2005 Nissan Altima. Im not real familiar with Japanese OEMs but I do most of my own repair work on my car (which currently has issues of its own) but I said Id help her out. Any ideas on whats wrong and how do I go about making the repair…
The problem could be the coolant temperature sensor for the gauge or the temperature gauge itself.
Tester
it could also be a lack of coolant. 
@Ottomotive117 you need to be more specific, please. Is the gauge all the way hot or cold?
I believe this is your coolant temperature sensor.
The coolant level is full. We rented a pressure tester and everything seems ok. How do I tell if it is the sensor? Where could I find it???
Look near the thermostat. Do you have a multimeter? If you do, you can use that link I sent you to test the sensor.
The nissan foru,d day it is on the back side of the head on the drive belt side. Of course you didn;t say which engine and neither did they.
2.5l and the gauge is pinner cold, and hasnt changed in a couple days…the car drives ok. no signs of overheating…
Once you find the sensor you could remove the wire and ground it briefly to see if the gauge goes to HOT. If there isn’t a change with the gauge when you do that then the wire may have a break in it or the gauge is bad. Make sure the sensor wire has good continuity back to the gauge and the gauge is getting voltage to it before replacing the gauge.