Yes, you need to replace.
The harness is designed to stretch up to 18% in a crash, as a ride-down dispersion technique. Putting straps through the washer pulls the elastic stretch out of the fibers before a crash, eliminating that safety element. Also, soaking and harsh cleaners (such as detergents) weakens the strength of the overall harness.
The manual does address it in all child restraints. Which seat do you have? (Make, model, year).
Originally Posted by mama~bear
Reading through some other posts and noticed a comment about not submerging harness straps in water or washing in the washing machine. I'm sure we've done this once or twice with our straps (after a vomit episode!).
I read my manual completely and don't remember seeing that this was not recommended. How bad is this? Do I need to replace and how? Sigh ...