Zanussi FJS1225W Stopping at E90 Error Code

Our Zanussi FJS1225W is putting up E90 fault codes, and just
once E91. When it does this instead of a fail-safe
shutdown of the machine it goes into fail-unsafe mode,
locking the machine into running whatever it happened to
be running when it put up the fault code. On one occasion
this left the pump running, and the last time the heating
element which nearly boiled the thing dry, ruining a clothes
load and melting the soap tray!

We have cleaned and tested the wiring to, from and
between the two boards, and examined and tested the
components on the main board as far as we were able.
Following recent main motor brush replacement, and the
commutator cleaned, the machine ran well for a few loads
before putting up E90. The main board had attracted a lot
of carbon which we cleaned off with a cotton bud and
meths.

We usually run the machine on economy 7 at night, but
can no longer trust it since the last ‘meltdown’, and need a
definitive fix for its seemingly built in fail-unsafe
mechanism, as we are quite keen to keep waking up in the
morning!

Is the fail-unsafe mechanism unique to Zanussi, or are all
washing machines this homicidal?

Any new ideas as to the likely fault much appreciated,
thanks.
Neil.