Hi I have a 2013 focus with the 3.5 inch screen. I was wondering if I could upgrade it to the 4.0 inch without having to replace the headunit. Bit of a novice. Thanks

This can be done without any problems as long as you install the correct one. Which version of the FCDIM (display) you need depends on the type of audio system (Low Level or High Level) and in case of a High Level audio system whether it is based on the old Bluetooth/Voicecontrol system or the SYNC 1.0/1.1 system.

It is also possible to retrofit the Original SatNav system. The SatNav functionality is completely integrated into the 5 Inch full color FCDIM and basically has nothing to do with the ACM (Audio Control Module). Both are standalone modules that communicate with eachother (and other modules) over the CANbus network.

I've got the sync 1.0 head unit. How do I check if it's high or low level. Will I need to   get a different facia for the buttons to fit the screen. Finally will I need to do anything in the cars computer. Thanks

SYNC 1.0/1.1 based systems are Always High Level audio systems. These can be identified by the 10 digit numerical control panel. 

You can install a 4 Inch full color FCDIM (which is used on the Sony audio system) or a 5 Inch full color SatNav FCDIM (which is used on both the Sony and standard SatNav audio systems).

The 4 Inch FCDIM can only be installed in combination with the corresponding fascia for the 4 Inch display. The 5 Inch FCDIM can either be installed with the corresponding fascia for the 5 Inch display or by making some easy modifications to the fascia of the 3.5 Inch monochrome FCDIM. SatNav functionality of the 5 Inch FCDIM can be controlled from within the audio system menu so it is not really necessary to install the SatNav control panel.

The type of display should Always be correctly configured in the Central Car Configuration. Without being configured properly the FCDIM will work but some functionality may be missing or not work correctly.