F18 is a drainage fault code.
I would start by switching of the washer, checking the pump filter and giving it a clean, and removing any debris from the void where the filter sits, then put the filter back in place.
Next I would check the waste pipe for any blockages (you will know if its blocked as it will be heavy), and then the drain pipe the hose goes into.
Once your happy they are all clear, I would run the washer on a rinse cycle and see if the fault reoccurs, if it doesnt proceed to a normal wash.
Should you continue to have issues, it likely you will need a replacement pump.
Please let me know how you get on.