You might have External IDs or External Course Numbers that consist of 5 digits. Instead of typing 00041, simply type 41 and let Excel add the leading zeros. CSV files are notorious for dropping these zeros when you save. Make sure you do this process and then do not open the file again. Each time the file is opened, csv will drop your settings and you will have to repeat the steps below!
1. Select column, right click, and then click Format Cells.
2. Select Custom.
3. In the Type: area, enter the following number format code according to the size of the field: 00000