It enters sleep mode when the pencil is placed back in the holder, so it's worth checking that. Make sure you have the holder cable connected, and if you already do, you can probably remove the end that goes into the base unit and do a continuity check on it to verify the switch is actually working. You can also check the manual for setting sleep functionality, and make sure your temperature is above 300C, as it states it will not sleep if below 300C.