Whirlpool DWF 406 error – four blinks

I’ve been having a problem with this machine. Towards the end of the cycle (I think just before draining) the machine stops and beeps on/off continuously. On opening the door, the LED next to the start button flashes in a four-flash sequence. After a reset (pressing down the start button for 2 seconds) the machine will sometimes complete the cycle. Yesterday I refilled the machine with salt (even though the salt indicator was not lit) and this seems to have solved the problem for now.

The instructions for use supplied with the machine are pretty useless, giving no information on these error codes (just the usual advice about cleaning the filter, which we do for every wash).

Last November I called out a Whirlpool technician when the same thing happened. Unfortunately I wasn not there when he called so I don’t know exactly what he did (although thre were no parts replaced). I really don’t want to spend another €100 on another visit, so doesn anyone know where there might be a list of what these error codes mean and whether there are any more technical instructions available for these machines?

I live in an area with very hard water. The machine is 3 and a half years old and is used intensively (i.e. about twice a day).

Thanks.