Hi Yanzhe,

There are several things that could cause your symptoms.  Please run the following test to help isolate the problem:
1) Swap the RA and DEC cable connectors *AT THE MOUNT*.  The yellow LED will remain on, this is OK.  Test N/S and E/W to determine if the problem follows the N/S buttons to the RA axis.  If not, the issue is in the mount.  If the issue follows the N/S buttons to the RA axis, continue on to the next step.
2) Now swap the RA and DEC cable connectors *AT THE CP5* and see if the issue returns to the DEC axis.

Regards,
Mike Hanson