I called to this last week for a 4 flash fault. The drain pump was in good condition, no obvious signs of any blockage. Set it to a programme and it filled, washed and drained perfectly.
She has rung to say it is still flashing 4 times even though it has emptied OK. 😕
I think from memory that I had one that the pressure switch in the sump was so full of fat that it wasn’t returning to the empty position within time, I wasn’t able to clean it out although you may be able to using a bit of hot water, I ended up changing it to cure the fault.
As you look down in the sump it’s to the right,bayonet fitting from underneath so not too difficult to take off and clean,plinth off and it’s tucked behind the sump hose.