Six Democratic US senators today criticized Comcast and other TV and broadband providers for charging erroneous fees, such as cable modem rental fees billed to customers who bought their own modems. The senators have written a letter to Federal Communications Commission Chairman Tom Wheeler asking the commission to "stop unfair billing practices."

Last year, more than 30 percent of complaints to the FCC about Internet service and 38 percent of complaints about TV service were about billing, wrote Senators Ron Wyden (D-Ore.), Bernie Sanders (D-Vt.), Jeff Merkley (D-Ore.), Al Franken (D-Minn.), Ed Markey (D-Mass.), and Elizabeth Warren (D-Mass.).

The senators described Comcast, the nation's largest cable and broadband company, as a repeat offender. "We are troubled upon hearing complaints of consumers being charged the modem rental fee after they have returned the rented equipment to Comcast or being charged the rental fee having never rented a modem in the first place," the senators wrote. "Not only are the majority of customers using automatic payment systems and may not personally authorize every erroneous charge, many consumers report having to call and remedy this problem throughout several billing cycles. In fact, customer help boards found online at Comcast’s Help and Support Forum contain complaints about this exact problem."

Comcast increased its monthly modem rental fee from $8 to $10 (in late 2014) "and makes an estimated $275 to $300 million a quarter from these fees," the letter said.

The problem is made worse by lack of competition, the senators wrote.

When contacted by Ars today, a Comcast spokesperson noted that "modems are available to consumers at retail, and rental isn’t required for broadband service."

The senators didn't make any specific call for an investigation but asked Wheeler several questions. The senators want to know if the FCC regulates erroneous equipment fees charged to consumers, whether the FCC has records or a database of erroneous equipment charges, how many consumers have complained to the FCC about incorrect equipment fees, and what action the FCC has taken in those cases.

Wheeler's office told Ars that "we’ve received the letter and are reviewing it." In most cases, it takes Wheeler at least a few weeks to respond to letters from lawmakers.

The FCC could take action against broadband providers using common-carrier regulation. Because of the FCC's reclassification of Internet providers as common carriers, the commission can intervene when customers complain about "unjust" or "unreasonable" prices and billing practices.

Wheeler has also proposed new rules to make the set-top box market more competitive, which might help customers avoid or reduce cable box rental fees.

UPDATE: Comcast offered an additional statement this afternoon, saying, “Comcast operates in a highly competitive environment across all of our lines of business. Acknowledging some of our issues with customer service, we have undertaken a substantial, multi-year effort to reinvent the customer experience, including improving billing systems, reliability, and all of our interactions with our customers. We are investing hundreds of millions of dollars in this effort and are working hard to improve and we won’t stop until we have made the changes necessary. We take every FCC complaint seriously, and respond to each one on an individualized basis. We are also using the information from complaints in our ongoing efforts to improve the overall customer experience.”