FrOd is referring to the external temperature sensor for a trip computer.

The engine temperature sensor is at the back of the engine under the inlet manifold above cylinder 1. Its a cylinder head temperature (CHT) so can be changed without any coolant loss.

However, if it is faulty, it would probably have effected the engine running as much as the indicator. The dials in a Focus are not independent devices. They are driven by digital highway from the PCM. In the case of the temperature it even adjusts the CHT to give a coolant temperature for EOBD.

Martin