Why isn't my heat guage working on my 99 ford ranger?

Why isn't my heat guage working on my 99 ford ranger? - 1

Two things to check. Is the engine actually warming up? If the thermostat is stuck open the engine won't warm up. This is bad for fuel economy and engine wear.

Or, the engine IS warming up (top of the radiator is too hot to touch). In this case you need to look at the temperature sender, the wiring, or the gauge itself.

Is it fluctuating is it frozen, have you checked the power and the ground to it

The first thing to do is check the coolant level in the radiator, not just the reservoir. A temperature gauge sending unit is only accurate if the cooling system is full. The find the temperature gauge sending unit and touch the wire that runs to the gauge to ground to see if the gauge goes all the way up. If it does and there's coolant in the system try a new gauge sending unit.