Citroen Xsara engine management light

Engine Management light came on four days ago and car became very sluggish when accelerating. Got a diagnostics test done on it which apparently showed up no fault at all and was told it was a winter thing and to take it back in a few days if it is still a problem. Well took it back today because I am still having the same problem and yet again the test showed no fault. Again I was told to basically learn to live with it and that the light was coming on because the fuel (diesel) was freezing in the pipe and as the car heat up the light should go off but this doesn't happen as the light doesn't come on until I have actually been driving it for a while. Anyone got any ideas why this is happening? Thinking that a second opinion is a good idea but was wondering if anyone else has experienced this and any advice you can give?