If I get a CD or D in my math class, do I have to retake it?

No, you don’t have to retake it. However we recommend that you do because these topics are important for your junior and senior engineering classes.

You will be using math, chemistry, and physics in your ChE classes to solve various engineering problems.  In particular, using math to solve engineering problems intensifies in the junior year. Students usually struggle with this, even if they have previously done well in their math classes because the problems look different, the terminology is sometimes different, and you’re being asked to recall concepts that you learned a year ago (or longer!).  

To prepare for junior year ChE classes, review topics from calculus, linear algebra, and differential equations. Review your notes from math classes, retake classes you did poorly in, and/or utilize online resources, like Khan Academy.

See also: What happens if I retake a class? and How can I succeed in ChE at Michigan Tech?