A faulty temp sending unit can do that.
normally the gauge should be in the middle.(between hot and cold) that is concider operating temp.
Engine is overheating, faulty gauge, faulty temp sensor......
temp gauge is stuck on hot
The temp sending unit may be shorted to ground. Unplug the sending unit to see if the gauge drops to cold. If it does, you have a faulty temp sending unit.
Too many Variables in play. Could be air in the system, faulty t-stat, faulty water pump, temp sensors etc. Gotta go over them all if you dont have an idea or can pin point any causes.
Test gauge function with a mechanical gauge to get actual engine temp.
This would indicate that the system is low on coolant when the gauge is hot. Not enough coolant to service the heater core.
Broken Temp guage
If you don't have a temp. light, or temp gauge, then just install one.
if it's all the way hot, the wire may be disconnected or shorted to ground, I forget which one makes the temp go high. Try disconnecting the sensor and see what the guage reads....it's based on resitence to ground, and I think if it's disconnected the gauge should read low. If you disconnect and the gauge does not drop, the wire may be "pinched" and grounded.....
Take wire off temperature sender look a guage. If guage at cool, temp sender needs replacement. If guage still at hot, temp sender wire shorted to engine or vehicle body.
thermostat may be sticking and that cause the gauge to show hot, have it checked soon, you could be headed for bigger problems