New to the forum so be gentle.

My daughter's Grande Punto 1.2 Active 06 plate, is showing fault code P0115, the coolant temp sensor. It's hard to start from cold and runs lumpy with management light up n.
I've changed the sensor today and it's still the same. There is 5V at the plug so I don't think it's a wiring fault. Once warm it's fine and the light goes out.
Any ideas what else it could be??
Any help much appreciated.