Two years after a 752-foot-long oil tanker collided in heavy fog with the Bay Bridge — an accident Coast Guard investigators blamed in part on a broken radar beacon on the bridge — one of the bridge’s three beacons is broken again.

The beacon, which alerts sailors to the midpoint between bridge towers, has failed to work properly since Dec. 17.

During those six weeks, hundreds of ships, including fully loaded oil tankers and massive container vessels bound for the Port of Oakland, have sailed under the section of the bridge with the broken radar beacon. Some of those voyages have occurred in heavy fog, increasing the risk of an oil spill similar to the 2007 accident that spilled tens of thousands of gallons of bunker fuel into San Francisco Bay — or a much worse disaster, like the 1989 Exxon Valdez spill in Alaska.

The repeated failure of the radar beacons, which are owned and operated by Caltrans, is raising concerns among environmentalists, the Coast Guard and the shipping industry.

“We have these technologies for a reason,” said Deb Self, executive director of San Francisco Baykeeper, an environmental group. “We want to make sure that pilots know where the bridge towers are, especially in heavy fog. Oil spills can cause enormous ecological damage to the bay.”

While they are sailing in and out of the bay, ship captains have radar, GPS, human lookouts on deck and radio contact with Coast Guard officials. But the beacons, which have been installed on the underside of the Bay Bridge since 1992 and send Morse-code-like signals that show up as marks on ship radar screens, are considered an important tool that many sailors rely on in bad weather.

“These beacons have been out for a significant time during one of the most treacherous periods of the year in terms of visibility and navigation. That is a concern,” said John Berge, vice president of the Pacific Merchant Shipping Association, an industry group.

“There are a lot of redundant systems,” he said. “But we should be using all the tools in the toolbox. The only really good oil spill response is never getting it in the water.”

The beacon that is broken is known as “RACON N.” It is located near the San Francisco shoreline between the western-most towers of the bridge, known as the “A” and “B” towers. The opening between those towers near the Embarcadero is the main lane for large ships coming into the bay and heading for Oakland, Redwood City or Anchorage 9, an area south of the bridge where ships anchor to refuel.

Caltrans officials said Friday that the Coast Guard first notified them Dec. 17 that the beacon wasn’t working properly. The next day, Caltrans crews closed a lane on the bridge and replaced the $40,000 beacon with a backup.

But two replacement beacons they installed also didn’t work properly. The third one is sending a weak signal and is listed as “inoperable” in Coast Guard records.

“It’s a mystery why the equipment is not working,” said Nader Eshghipour, Caltrans district maintenance director. “When you are working with equipment that is both hardware and software, the environment and the weather is a factor.”

Earlier this week, Caltrans crews sent one of the broken beacons to its manufacturer, Houston-based Automatic Power, for warranty repairs, as Caltrans electricians scurry to identify the problem.

Eshghipour said that the three Phalcon 2000 beacons on the bridge are inspected at least once a month by Caltrans crews in boats. He said since the Overseas Reymar accident on Jan. 7, 2013, there have been “three or four” other malfunctions of the beacons on the Bay Bridge, although Caltrans was unable to provide dates and durations of the outages by late Friday.

Ship collisions with the Bay Bridge have become a major issue in recent years.

In November 2007, the cargo ship Cosco Busan hit a bridge tower, spilling 53,000 gallons of bunker fuel into the bay, oiling 69 miles of shoreline and killing more than 6,000 birds.

Unlike the Cosco Busan, the Overseas Reymar was empty, having unloaded millions of gallons of crude oil at the Shell refinery in Martinez the night before. Had the ship been full of oil, it could have created an ecological disaster.

All large ships are guided through the bay by bar pilots who board as the vessels are entering and leaving the bay. The pilots are expert captains themselves who know the local conditions. The pilot on the Overseas Reymar was Guy Kleess, of San Francisco, 62, a former Exxon oil tanker captain who had sailed professionally for 36 years.

A final Coast Guard incident report released earlier this month found that Kleess had planned to sail between the Bay Bridge’s “C” and “D” towers in dense fog. But when he noticed the familiar “RACON B” beacon was not working, he made a last-minute decision to try and sail between the “D” and “E” towers closer to Yerba Buena Island.

He misjudged the turn, and in strong ebb currents, slammed the ship into the “E” tower, causing $1.4 million in damage to the tower and carving a large gash in the side of the ship. State regulators suspended his license for five months without pay, required more training and put him on two years’ probation.

The Coast Guard’s final report said Kleess was to blame for “numerous lapses in judgment.” It added: “The failure of RACON “B” played a significant role in this casualty.”

The report recommended that Caltrans “develop policy and procedures for testing, inspecting and monitoring” the beacons, along with notifying the Coast Guard immediately when they are broken.

Eshghipour said Caltrans does not have a statewide policy since the beacons are a local issue, but it has stepped up inspections and record-keeping since the accident.

Dan Dewell, a Coast Guard spokesman, said the agency is “very concerned” about making sure that all navigational aids are working well in the bay, but it has confidence in Caltrans.

“Caltrans is doing an excellent job of trouble shooting and trying to resolve the issue,” Dewell said.

Paul Rogers covers resources and environmental issues. Contact him at 408-920-5045. Follow him at Twitter.com/PaulRogersSJMN.