You are right about the Guarantee, however you have repair any sub-standard work promptly. If the customer doesn't want to wait, you're getting back-charged. Mainly because if you do shoddy work (not up to spec, code, etc.), then you are in direct breach of contract. If you are in breach of contract (even in Illinois), the burden of proof is on you.
This is CABL.com posting #165781. Tiny Link: cabl.co/mRh3