Dear Liz: Why is there such a difference between my FICO 4 and FICO 8 scores? My FICO 4 score is 646 while my FICO 8 score is 678. I want to buy a home and I know some lenders may still use the FICO 4.

Answer: Most (not just some) mortgage lenders use outdated versions of the FICO credit scoring formula. The agencies that buy most mortgages, Fannie Mae and Freddie Mac, accelerated acceptance of credit scores in the mid-1990s when they made FICOs part of the underwriting required for the loans they purchased.

But the agencies haven’t authorized lenders to use the newest versions or alternative scores, such as VantageScore. So an old collection or other misstep that’s ignored by modern versions of the FICO formula could hurt your efforts to get the best rates and terms on a mortgage.

There are several ways you can boost your scores in the coming months. First, get your actual credit reports from all three credit bureaus at www.annualcreditreport.com. (You don’t need to provide a credit card. If you’re asked for one, you’re on the wrong site.) Scan the reports for errors, such as accounts that aren’t yours or late payments showing when you paid on time. Dispute those and prepare to follow up with any creditors that insist on reporting false information. (Complaints to the Consumer Financial Protection Bureau can help you get the creditors to cooperate.)

Make sure you’re making all credit account payments on time and pay down any credit card balances. Your goal is to use 10% or less of your reported credit limits, and to pay your balances in full each month. (Homeownership is expensive enough without dragging costly credit card debt into the financial picture.) It may take a few months to start seeing improvements, but they should come.

When you’re closer to pulling the trigger on a home purchase, consider buying your FICOs for all three credit bureaus from MyFico.com. In addition to the FICO 8, which is the one other creditors use most often, you’ll get your FICOs for the mortgage, credit card and auto loan industries, which can give you a clearer picture of where you stand.