As above don't go head first into replacing the DPF without getting it diagnosed properly.

Some of these Focus have a tank under the back that has an additive fluid for the DPF and if it's empty can cause a fault/limp home mode, there is a Ford filling tool for this that dealers have.

If it's soot value is too high it may need a static regeneration where the ECU holds the engine at a high rpm with the vehicle stationary adding excess fuel to burn the soot out.
This requires the dealer tool to do this (if others can do it I don't know as only use Ford IDS)

The DPF learned values can be reset with the Ford dealer IDS tool, you may find an independent with it - whether any other types of diagnostic tool can do this I'm not sure , as only use Ford IDS at work.

I recently replaced an engine & turbo on a Transit TDCI 2011. It had oil pump failure, in turn turbo seized dumping oil into the DPF, it was that blocked it wouldn't run for longer than 5 secs unless the 02 sensor was removed from the front pipe thus venting the exhaust. New GENUINE DPF for the Transit - £1033 + VAT!