Well, for example if this a a credit card debt your contract may have waived service and submitted you to CA court jurisdiction when you signed up.
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You really need to address this question to a California attorney. If you were not served properly in most states judgments are void or voidable but you will have to go to the state court where the judgments were entered to undo them, and the procedures should be statutory. What you want to watch out for is the registry of the judgments here in Illinois because that will complicate things and could require action in both California and Illinois.Ask a similar question