I've heard many people in Singapore are talking their work like this way:
"Oh, I have some codes checked in, please find the error list of the error code that this service will return."
I have to say, when you are talking about the program that you have worked out, you should treat the noun "code" as uncountable, and always use single form of the term "code," for example:
"I write two lines of code to resolve this issue."
Meanwhile, when you are referring a specific unreadable string that represents a status, an exception or other meaning, should treat the noun "code" as countable, and choose the appropriate single or plural form of the term. For example:
"I don't think these two error codes make sense, one code 0x8000000f is reserved by Windows and is well known."
Hope this helps.