“The impact of inaccurate provider directories on consumers can be devastating, especially on those consumers who need to carefully examine networks for specific subspecialists, cancer centers or children’s hospitals,” the American Medical Association told state insurance officials in a recent letter endorsed by dozens of health care provider and patient groups.

But insurers say that the problems might not be easy to fix, and that doctors are partly to blame for the directory errors. Insurers “are unable to guarantee the accuracy of the provider’s status” in a directory because doctors often “stop accepting particular health plans’ members off and on throughout the year and fail to notify the plan in a timely manner,” America’s Health Insurance Plans, the chief lobby for the industry, said in a letter to the Obama administration.

In its online doctor directory, Blue Cross and Blue Shield of Texas says that it makes every effort to provide correct information, but that it “cannot be responsible for omissions or errors in the provider details.” Aetna says that data in its directory is “subject to change at any time.” UnitedHealth tells Medicare beneficiaries, “A doctor listed in the directory when you enroll in a plan may not be available when your benefits become effective.”

The problems that consumers face with unexpected costs may result, in part, from the way plans are listed on HealthCare.gov, the website for the federal marketplace. More than 8.5 million people are in private health plans selected through the site, and the plans are listed in order of their premiums, from lowest to highest.

This encourages consumers to focus on premiums rather than total costs, said Mr. Krughoff, the Consumers’ Checkbook president, and they often spend hundreds or thousands of dollars more than they need to.

Mr. Krughoff’s group has been publishing a guide to health plans for federal employees for more than 30 years, and a version of its online tool for comparing health plans is available on the website of the federal marketplace in Illinois. “It’s been a great tool,” said Jose M. Muñoz, a spokesman for Get Covered Illinois, the state agency that promotes enrollment.

The tool can perform searches tailored to a person’s needs and priorities. It asks consumers to describe their health status, offering five levels from excellent to poor, and to list “expected medical procedures” like childbirth, knee replacement or prostate removal. It also provides an estimate of total yearly costs for the user.