Even if we’re not quite sure what goes into our credit report or how that information boils down to a three-digit score, we know that a higher number means a better shot at getting a credit card or mortgage loan, along with a better interest rate on money we borrow. But many Americans don’t know about another type of credit report that’s just as important — in fact, a poor score can lock you out of the financial mainstream for years.

It’s called your ChexSystems report, and it tracks certain behaviors and activities related to checking or savings accounts. Bounced a check? That goes in the report. Had an account closed by your bank after you ran it into the red? That does, too. These black marks stay on your record for five years, and can keep you from opening a bank account. According to an FDIC-commissioned study, 25% of banks won’t let you open an account with them if you have even a single derogatory item.

There are a couple of other companies that also track this kind of information, but the largest is Chex Systems Inc., which is part of Fidelity National Information Services, a big player in the financial services industry. It serves around 80% of banks in the United States.

While nobody’s saying serial bad-check writers should get a free pass, consumer advocates say there are some problematic elements in the way ChexSystems operates, starting with that one-strike-you’re-out standard that many banks use.

In a blog post on CreditSlips.org titled “Secret Creditor Reports, Overdraft Fees and the New Unbanked: The Perfect Storm,” University of New Mexico law professor Nathalie Martin says, “Nearly 90% of financial institutions use ChexSystems or similar reports in their account opening process, yet they are under no duty to disclose this to consumers until an account is denied due to information contained in the report. For those consumers denied accounts, it is too late.”

“The first problem is that they’re only a negative blacklist,” says Ed Mierzwinski, consumer program director at U.S. PIRG. Nearly all items contained in a person’s ChexSystems report is derogatory, he says, “But people don’t know about it because they’re so under the radar.”

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Since many people don’t know about this report until they try — and fail — to open a new account, this knowledge gap could leave them stranded in unbanked limbo if they had already closed the old account the denied one was intended to replace.

This is a pricey pitfall, since these people are likely going to be relegated to products and services like prepaid debit cards and check-cashing storefronts until they can find a bank that will let them open an account, or until the item comes off their report.

How do you avoid being one of these unlucky customers? Under the Fair Credit Reporting Act, everyone is entitled to a free copy of their ChexSystems report every year, but it can’t be obtained from the central credit report request site annualcreditreport.com, where you can get reports from Experian, Equifax, and TransUnion. (It can be requested from the company directly here.)

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If you find a mistake, Chex Systems has a system in place for customers to dispute errors on a report. Paul Stephens, director of policy and advocacy at the Privacy Rights Clearinghouse, says anecdotal reports indicate this can be easier said than done. But if you file a dispute and it isn’t resolved to your satisfaction, he says, “you do have a right to place a consumer statement on your report if you disagree with something that appears in your report” — though that’s not a guarantee the bank will be more lenient.

“ChexSystems reports may be accurate, but that doesn’t make them fair,” Mierzwinski adds. For instance, he says, if you close an account but a final debit accidentally puts your account into the red — probably generating overdraft fees — you might have no idea if the bank doesn’t contact you.

In 2011, Chex Systems Inc. settled a class-action lawsuit related to its handling of disputes. According to CNN Money, “ChexSystems denied the allegations but agreed to pay each class action member $82 per valid claim — up to a total of $278,000 for approximately 3,400 potential claims.”

If you plan to move your banking to a new institution, Mierzwinski recommends keeping the old account open and funded for several months after you’ve set up the new account, to make sure any pending checks or debits have time to clear.