A new BART train failed a key safety inspection Friday, possibly preventing it from becoming operational before its Thanksgiving target.

When the 10-car train arrived at the Bay Fair station as part of a California Public Utilities Commission, or CPUC, inspection, it unexpectedly entered a safety mode. The controls switched to functioning as those of a three-car train, preventing the rear seven doors of the train from opening, according to BART spokesperson Jim Allison.

The cause of the error is still unknown, Allison said, adding that BART hopes to have the 10-car train operational before Thanksgiving.

“So far, it is too early to tell whether we will be able to do that or not,” Allison said.

The trains have onboard diagnostic equipment, which is being investigated to determine the source of the error.

The safety mode was designed for emergency situations, such as if a car door opened while the train is in transit, according to Allison. The passengers in the rear seven cars would move into the first three cars to exit. The first three cars would remain fully operational because they are closest to the train operator, who is a trained first responder.

This 10-car train is the first of a new fleet of 775 cars that will be operational within the next five years, Allison said.

The new cars will replace the current BART fleet, which has been in service since 1972 and is exceeding its designed operation age. If the trains continue to operate in their current state, the outside of the trains “would actually start to rip,” he added.

The manufacturer of the new trains, Bombardier Inc., has not manufactured any previous BART trains, Allison said. The new train model has successfully operated in five-car trains, but “putting everything together” resulted in the error Friday, according to Robert Raburn, vice president of the BART Board of Directors.

CPUC is responsible for approving BART trains and “only really (comes) into play” when BART is launching new vehicles, Allison said. In 2014, when BART launched its line connecting the Oakland International Airport and the Coliseum stations, CPUC did not detect any problems.

BART’s order for the new fleet will not change based on the failed inspection, according to Raburn. Allison added that the results of the inspection are “relatively minor.”

“It would be a disaster of the first magnitude to even hint that there would be changes to that car order,” Raburn said.

After BART makes necessary modifications to the train, another test run will be required, according to a letter sent to BART on Monday by CPUC.