One problem related to o2 sensor has been reported for the 2013 Ford Focus. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2013 Ford Focus based on all problems reported for the 2013 Focus.
As I was driving the car, at low rpm's the car would stutter and stall. It is more pronounced with the air conditioning on. Took it to the dealer and could find nothing wrong with it, I was told this is common. On my way home from the dealer the car stalled leaving a green traffic light and I was almost hit in the rear end. The car finally lite a check engine light 3 weeks later. Dealer changed an O2 sensor and 4 days later in July car went into "limp mode" in first gear as I was leaving a restuarant. The car hesitated and stalled in traffic, I was almost hit in the rear of the car again. The dealer claims that the an actuator controlling the radiator vents is bad, but this should no have caused the car to go into limp mode. Ford has a TSB for a wiring harness problem, the number is TSB 13-9-7 that explains the issues of stuttering and stalling, but unless 2 codes are read on the computer they refuse to fix it. The problem is well documented on Fordfocusst. Org website. Hundreds, if not thousands of cars have the same issue. I believe that one day someone, if not many people, will be seriously hurt due to this issue.