Thermostat circuit
 

I have a 98 Cherokee xj with the thermostat gauge reading random temps. I already replaced the thermostats and ran the code and it said low thermostat circuit voltage and low charging output. How could I fix it?

The problem isn't the t-stat. It's either the sensor (likely) the wiring (quite possible). The sensor can be tested. Just search YouTube for "test cherokee temperature sensor" and you'll find plenty of videos.

If the sensor seems okay, then start looking at your grounds.