Warning Message pop up - without data validation

[rk777]

Hello,

I am posting here for the first time. Have been following this website for excel help for a while now. I have a project where I am trying to make a Warning or Stop msgbox pop up if a cell does not equal to 0. Here is what I am trying to do:
.....A......B
1 Active Code
2...x ........3
3...x ........2
4...x ........5
5...3 ........3 ...........In cell C5 I put this formula =A5-B5 = 0

I have two columns. If a user puts an x in the active column, they must select a reason code(from a set list) in the Code column. The total is done in each column using a countif function. In cell C5, I put a formula to do a subtraction(Active-Code) of the totals in each column. C5 needs to always be 0. If a user did not put a code or did not put an "x" in the Active column then C5 will not show 0, but a positive or a negative number. When that happens I need a Warning box to pop up to inform the user they made a mistake and they need to go check their data.

Could someone please help?

Thanks!