A coach was evacuated on the High Street after billows of smoke started coming from its engine.
Firefighters rushed to the X5 bus and found the large amount of smoke was caused by oil dripping on to the hot engine.
A dozen passengers were evacuated from the bus, which travels between Oxford and Cambridge, but no one was injured.
Two fire crews, from Buckingham and Bletchley, cordoned off the road for half an hour while the engine cooled down, close to the Thirteen High Street pub.
Watch commander for Buckingham Fire Station, John Robinson, said: “It was an overheated engine and there was a lot of smoke but we made the scene safe.
“The driver did an excellent job of evacuating people. The fire sensor was activated and he did the right thing of pulling over.
“It was a mechanical failure and that can happen to any bus – this one is ultra-modern.
“The engines are actually designed to run hot, because it makes them more efficient.”