Sounds like an immobiliser fault, likely to be caused by bad soldering in the dash cluster (speedo clocks), fairly common issue on the Mk2 although less frequent on the Mk2.5. If you have a Windows laptop I'd recommend Forscan over any generic OBD scanner.
It puts the Engine systems light on (the one with an ! in a cog) also on my car. 1.8 TDCI Mk2 Focus Ford Year: 2006 UK/Ireland Location: 2012 Galaxy Titanium
Ford Focus: I have Mk2 Focus 2.0 tdci engine systems fault… I have Mk2 Focus 2.0 I have Mk2 Focus 2.0 tdci the engine systems fault warning message came on after filling with fuel.MIL lamp not illuminated and engine not in limp mode.Suspect DPF fluid.Will I need full diagnostic?
Cant comment on the engine systems fault as that usually shows a code to go with it but it's also worth just checking the fuel filter. Drain it if it has a drain to check for water. Renew it if your brave as there is no bleed mechanism on this engine and you need to draw the fuel up from the tank after changing it.