Temp gauge low..low or broken..

[MildSeven]

Hi, I'm sorry if I'm posting at wrong area.
it's about my gf's car, 1991 toyota corolla.
It's not my car but one thing bugs me everytime I drive her car.
The temp gauge on the cluster never go up. maybe 3%-10% and again it goes down. What can it be? thermo sensor? Coolant system? (the heater is working fine)

[BradE]

The first thing to replace is the thermostat. They typically run less than $10 and are pretty easy to replace. It sounds like your thermostat is stuck open, causing constant flow of coolant and never allowing it to reach normal operating temp.

[MildSeven]

thanks for your help it does bug me but is it necessary to fix?

[BradE]

Well, if the thermostat is bad and the engine is running cold it causes the car to run rich. That's a bad thing. It can cause premature failure of the O2 sensors, cats, lower MPG, etc.