Can a bad thermostat in a car cause the tempature guage in the dash to not work? Or is the guage itself broke?

I have a 1991 Ford ranger and have no idea if the engine is overheating because the gauge doesn't work. Thermostat or gauge? Of its the gauge do I have to replace the whole cluster?

There's a temerature sensor located on the front of engine… Usually in front of the thermostat housing on the V6 rangers. Now, be careful, there's 2 sensors with the same wiring harness connectors on those engines. 1 is a coolant temperature sensor that sends info to the computer and the other is simply a temp sensor that sends a signal to the guage. Check out this sensor first. If it isn't the temp sensor, then the wiring should be checked going up to the guage and if it's the last thing that needs replaced, then the guage can be replaced without replacing the whole guage cluster.

The thermostat has nothing to do with the gauge. It could be the gauge is bad, or the sending unit, or the wiring in between. You might also check the fuse that gauge operates off of.