One problem related to o2 sensor has been reported for the 2011 Ford Fiesta. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2011 Ford Fiesta based on all problems reported for the 2011 Fiesta.
Took car to mechanic to have O2 sensors replaced at time of oil change. Auto shop completed oil change and changed O2 sensors, reset check engine light, but then car wouldn't start. Mechanic reassessed, found blown fuses in fuse box, noting dirt in fuse box and corrosion. Car was towed to Ford dealership for further diagnostics as auto shop stated they couldn't do anything for Ford ecu. Ford ran detailed diagnostics to again find that dirt and corrosion in fuse box led to malfunctioning and car not starting.