My machine keeps displaying a E9 fault, what should I do to repair this?

You have not mentioned the make and model. Now the error is an indication of the fault or a protection indicated.
So remove power and plug in after making sure the door is shut - the door switch is checked to work.
If the error comes on , check on the power to the control panel, observe board for visible damage, check the inlet valve, check for working, check the drive motor, capacitor and drain pump/valve. Once confirmed that all these are working check for line break , if there was damaged done by rats/insects.
If none of these show an indicative fault then the program controller can be faulty.