The arithmetic, stated once and completely, because the order of operations is the whole point. The lender takes possession, incurs costs getting the property back and preparing it for sale, sells it, and applies what is left after those costs to the loan. Published material on auto loans states the rule: the reasonable expenses of retaking, holding and disposing of the collateral are paid out of the proceeds before the debt itself, with the borrower liable for any deficiency that remains. The consequence people miss is that the deficiency is not the difference between the payoff and the sale price. It is that difference plus the expenses, because the expenses were taken out of the money that would otherwise have gone to the loan.
A deficiency balance is not a deficiency judgment. The balance exists as soon as the sale falls short, by operation of the loan contract and the security agreement. A deficiency judgment is a court order entered later, if the lender sues and wins, and it is what gives the lender the enforcement tools: wage garnishment, attachment of a bank account, a lien on real property. A deficiency balance does not have to become a judgment at all: a lender may charge the account off and sell it instead, and a debt buyer may collect on it without ever suing. Published material on civil judgments covers what a judgment makes possible.
What the balance becomes is the reader's real question, and the answer is that it changes category. Secured debt that has lost its collateral is unsecured debt. Four consequences follow, each already covered elsewhere on this site and named here so the reader knows where they now stand. A third-party collector pursuing it is subject to the federal collection rules. The limitations period is a defense that has to be raised rather than something that happens automatically. If the lender eventually forgives the balance, the forgiven amount is generally income. And enforcing it against wages or an account requires a judgment first.
The credit-reporting clock does not restart, and this is the fact most often stated wrongly. 15 USC 1681c(c)(1) provides that the seven-year period "shall begin, with respect to any delinquent account that is placed for collection (internally or by referral to a third party, whichever is earlier), charged to profit and loss, or subjected to any similar action, upon the expiration of the 180-day period beginning on the date of the commencement of the delinquency which immediately preceded the collection activity, charge to profit and loss, or similar action." So the clock runs from the original delinquency. The repossession does not reset it, the sale does not reset it, and selling the account to a debt buyer does not reset it. Published material on credit reports carries the rule and the rest of the table.
Whether the lender may pursue the deficiency at all is state law, and this page will not give a national answer. Repossession and sale of personal property proceed under article 9 of the Uniform Commercial Code as enacted in each state, which sets what notice is owed, whether the borrower may cure or redeem, and what a lender must do to preserve a claim to the shortfall. For real property, published material on foreclosure records the honest structural position: some states bar the lender from pursuing a deficiency in some circumstances, and some restrict it by reference to the property's fair value rather than the price it fetched at the sale. Those differences are real and they are not summarizable in a sentence that holds everywhere.
One product has no deficiency at all, by federal definition. A non-recourse pawn loan under 12 CFR 1041.3(d)(5) requires that the lender's sole recourse is keeping the item, and the rule's commentary removes the exclusion if any consumer, co-signer or guarantor "is personally liable for the difference between the outstanding balance on the loan and the value of the pawned property." That difference is a deficiency. It is worth knowing that such a structure exists, because it makes clear that a deficiency is a feature of the contract rather than an inevitable consequence of losing collateral.