how can i fix my zanussi zwf1621s?

seems to go through wash/rinse cycle ok until spin, then all the lights on the front panel flash and eventually it stops and wont pump out .
The manual explains this as imbalance sensor being activated but the machine has never done it before, nw does it allthe time .The machine is very smooth and never seems to tangle or imbalance.Is the sensor broken or faulty in some way?