1997 Honda Civic 97 Honda Civic, temperature gauge reads hi

I am trying to fix my sister’s 97 Honda Civic that she had purchased a few months earlier. It’s got 140k miles but is in very decent condition. About a month ago, we replaced the O2 sensor but this week she complained about the temperature gauge going up within few minutes. It stays up regardless you turn A/C or heater on. Coolant level isn’t low; car doesn’t smoke nor has coolant smell. I removed the thermostat, refilled the radiator to no avail. Cooling and A/C fans work fine. What else could be wrong? cooling temperature sensor? Just want to make sure before I run over to AutoZone. Thanks.