The company that runs the hotels, restaurants, campgrounds and shuttle buses at Yosemite National Park is failing to meet the park’s customer service standards and faces possible financial sanctions, newly released park documents reveal.

Performance reviews of Philadelphia-based Aramark corroborate complaints that visitors have increasingly had about food, campground reservations and, most extensively, shuttle service at one of the nation’s busiest and most venerated national parks.

The shuttles, which ferry millions of people each year to Yosemite’s storied waterfalls and mountain vistas, simply haven’t kept up with demand, the latest review said. Visitors commonly wait 45 minutes for rides, according to the document, and frustration over the shuttles has led to fiery exchanges between drivers and passengers, putting a black mark on many vacations. On one occasion, a group of park visitors formed a human chain across the road to halt a bus that was too full to stop.

Earlier this year, The Chronicle documented the problems with the shuttles, although the National Park Service hadn’t provided requested reviews of Aramark’s performance at the time. The newly released reviews and associated documents, furnished under the Freedom of Information Act, fill out the picture of the company’s shortfalls and bring to light disciplinary action the park is threatening.

In a March 4 letter to Aramark, Yosemite Superintendent Michael Reynolds warned the company that it must improve services by June 1, and maintain the higher level of service through the year, or the park would not grant its requests to raise rates charged to visitors.

Aramark, like concessionaires at other national parks, needs park permission to charge higher prices to cover costs. Park officials did not speculate how much Aramark stood to lose, but it’s believed to be significant.

According to park spokesman Scott Gediman, Aramark improved its performance enough — at least so far — to avoid the sanctions. The biggest step forward, Gediman said, is increasing the number of visitor shuttles that run in congested Yosemite Valley.

The Park Service, though, is still monitoring the company’s work closely.

“Aramark has been responsive and made a concerted effort to improve,” Gediman said.

In an email to The Chronicle, the company said its “full attention” is on the shuttle service and other issues raised in the latest review, which covered the company’s performance in 2018.

“An operation of Yosemite’s magnitude is highly complex, and we work hard to ensure compliance with our contractual obligations,” wrote Aramark spokesman David Freireich. “The opportunity to serve as a steward of Yosemite is a responsibility we take very seriously, and we remain focused on creating memorable and enjoyable experiences.”

Aramark, which manages hotels, restaurants, sports venues and other visitor services around the world, won the contract to manage Yosemite’s concessions three years ago. It’s one of the most lucrative contracts in the National Park Service, valued at $140 million annually, and positions the contractor and its more than 1,000 employees in the park as the face of Yosemite for nearly 5 million visitors each year.

The contract runs through 2031.

The performance reviews of Aramark suggest the company met expectations in its first two years at the park, when it made big improvements to food facilities, including upgrades to Yosemite Valley Lodge Base Camp Eatery and Degnan’s Kitchen, and continued repair work at the famous Ahwahnee Hotel. On a rating scale of 100, the company scored 85 in 2016 and 89 in 2017.

In 2018, however, Aramark’s score slipped to 69.

Superintendent Reynolds said in his letter to the company the rating reflected “failure for the concessionaire to substantially meet visitor service standards.”

The review highlighted problems with food service at the newly renovated Base Camp Eatery and Degnan’s Kitchen, where customer surveys showed satisfaction rates were well below 50%. The review did not spell out the specific problems. Incidentally, many complaints, according to the document, were about Aramark’s decision to allow a Starbucks to open in Yosemite Valley, a move that others have praised.

The review also noted problems that visitors had making reservations at the High Sierra camps and at Housekeeping Camp. It documented discrepancies between prices listed online and actual rates at such facilities as the Ahwahnee Hotel and reported that at some places, visitors were overcharged for services such as raft rentals and horseback rides.

The biggest issue, though, was the shuttle service. Designed as a way to alleviate traffic in the park, fleets of mostly diesel-electric hybrid buses offer free rides to visitors as an alternative to driving their cars, often going to places that don’t have parking.

The park’s review of the company said it didn’t have nearly enough buses on the road last year, noting that too few drivers had been hired and as many as 10 coaches were out of service because of a similar lack of mechanics.

Earlier this year, drivers said that while buses were inspected daily, there was pressure to keep them on the road and that intermittent problems like broken air conditioners, horns and turn signals often emerged. Vehicle registrations and fire extinguishers also were not always up to date, they said, and many questioned the safety of the fleet.

Gediman, the Yosemite spokesman, said Aramark had recently fulfilled the park’s demand of getting 13 buses running on the 8-mile Yosemite Valley loop. As few as six shuttles had been operating during peak periods previously, according to drivers.

“We’re working with them literally on a daily basis to make sure good service is achieved,” Gediman said. “This is something we take very seriously.”

Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander