Had the same problem on mine. Turned out to be a broken fan. Easily ordered on-line (£25-£30). 10 minutes to fix. The fan is in the top of the freezer compartment. Take out ice cube maker, rear panel comes off with 1 screw and there's your fan. Simple. Sensor is there too if it does need changing, but much more likely to be burned out fan.
Doug, August 2011