Hi,
Have you been fording in mud recently? It can cause issues when it gets behind the disk. As Johannes says also try adjusting the sensor, tap it in a little, but do it with the wheel off so you can rotate the disk and make sure the sensor is not touching the agitator ring. Also check the sensor cable it may have been damaged. If you have a diagnostic tool also check the error codes. Sometimes disconnecting the battery and rebooting the ECU can do wonders.
Thanks my advice anyway.
