Here's a list of some of the most common FTP codes you might see:
100 Codes The requested action is being taken. Expect a reply before proceeding with a new command.
110 Restart marker reply.
120 Service ready in (n) minutes.
125 Data connection already open, transfer starting.
150 File status okay, about to open data connection.
200 Codes The requested action has been successfully completed.
200 Command okay.
202 Command not implemented
211 System status, or system help reply.
212 Directory status.
213 File status.
214 Help message.
215 NAME system type. (NAME is an official system name from the list in the Assigned Numbers document.)
220 Service ready for new user.
221 Service closing control connection. (Logged out if appropriate.)
225 Data connection open, no transfer in progress.
226 Closing data connection. Requested file action successful (file transfer, abort, etc.).
227 Entering Passive Mode
230 User logged in, proceed.
250 Requested file action okay, completed.
257 "PATHNAME" created.
300 Codes The command has been accepted, but the requested action is being held pending receipt of further information.
331 User name okay, need password.
332 Need account for login.
350 Requested file action pending further information.
400 Codes The command was not accepted and the requested action did not take place.
421 Service not available, closing control connection. (May be a reply to any command if the service knows it must shut down.)
425 Can't open data connection.
426 Connection closed, transfer aborted.
450 Requested file action not taken. File unavailable (e.g. file busy).
451 Requested action aborted, local error in processing.
452 Requested action not taken. Insufficient storage space in system.
500 Codes The command was not accepted and the requested action did not take place.
500 Syntax error, command unrecognized. This may include errors such as command line too long.
501 Syntax error in parameters or arguments.
502 Command not implemented.
503 Bad sequence of commands.
504 Command not implemented for that parameter.
530 User not logged in.
532 Need account for storing files.
550 Requested action not taken. File unavailable (e.g. file not found or no access).
552 Requested file action aborted, storage allocation exceeded
553 Requested action not taken. Illegal file name.