A low coolant level could cause the temp gauge not to read correctly. A faulty temp gauge sending unit could also be the cause. The coolant leak, you would have to find the source of the leak to determine the cause.
In front of the radiator
Because the temp gauge is not giving you the temperature of the water/steam in the radiator. You may have a bad head gasket or a cracked head causing the coolant to have that boiling effect.
on the side of the radiator
i just put in a new radiator and now temperature gauge reads high but their are no leaks
temperature gauge not working can be caused by a bad temp sensor in engine, a bad gauge in dash, or a bad fuse
no you need to replace temperature sinding unit
The car can begin to overheat because it is low on oil or antifreeze/coolant. Your radiator can also be acting up. It is best to address this as soon as possible and not to let it go.
The symbol used on the Renault Scenic for a problem with the radiator is a thermometer. There should also be a temperature gauge on the dash that shows the current temperature of the radiator fluid.
You need to check the radiator and be sure it is full. It can be empty and the overflow tank full. If it is full then check and be sure the radiator cooling is working when the engine is at operating temperature.
Broken Temp guage
Head gasket. Do block test for gases in coolant