He soon signed for Birmingham City, where he helped the club to the 2001 League Cup Final. In the quarter finals he had given his old club some food for thought, scoring in a 2–0 victory over the "Owls" on 12 December 2000. In the final he was replaced by Bryan Hughes after 71 minutes and was forced to watch his team lose the penalty shootout to Liverpool 5–4, after a 1–1 draw in normal time. In a play-off semi-final defeat at Preston North End's Deepdale, Sonner, teammate Michael Johnson and boss Trevor Francis were all charged with misconduct by the Football Association, Sonner and Johnson were also charged for using abusive language. Sonner was fined £1,000 following a hearing.