This means that the battery is low. Please change it following this procedure:

  1. Switch off tamper protection (this process can be found here in the installation manual on page 9, Table 1.) link to manual found on this page http://www.yale.co.uk/en/yale/couk/productsdb/alarms/-ef-series-alarms--accessories/Standard-Alarm-Kit/
  2. Unscrew the two Key Pad case screws and remove Key Pad back to reveal battery.
  3. Using a screwdriver gently lever out the old battery.
  4. Insert new CR2032 coin cell battery with the + side uppermost
  5. Press battery into holder firmly with finger and thumb until a click is heard
  6. Press a number key and check that the LED lights. If the LED lights the new battery installation is successful, screw the Key Pad case back on and the battery change is complete.