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The majority of the time for a misfire to happen it could be a number of things, a failed coil, wire or spark plug. A compression issue with the engine which is caused by internal engine issue. A failure or issue with the injector for that cylinder. An 02 sensor will not cause a misfire. It is rare that a vehicle will have multiple failures like this unless the vehicle has been operated for a long time with the issue. By the sounds of it they misdiagnosed the issue.