However, there may be a more basic problem. If you are trying to use the UCC as a federal question cause of action, it does not qualify. Thus I see only two state law claims in your hypo. While a state law claim CAN serve as anchor claim, it has to satisfy 1332. Need more facts.
P discovers a malfunction.
D refuses to refund or replace the computer. I don't know what you mean by violating the Statute of Limitations. Then we could analyze how closely related the two claims are. Hope this is helpful.