So you want to use the answercode instead of the answer for your equation? Then you need to address the question correctly in the EM. usually if you address it with the question code, you should get the answer code. If you use questioncode.shown, you get the actuall answer as it has been visible to the respondent. Have a look at the EM manual:
www.limesurvey.org/manual/Expression_Man...efits_of_using_EM.3F
Point 6 should be what you are looking for. I recommend to read through the whole page though, because it will give you quite a good idea of how EM works and it gives some good examples to understand things better.
You might need questioncode.code. But for me, if I use the question code, it usually gives me the code anyway.
Maybe you can give us an example of what you are trying to do (how your questions look like and how your expression looks like).