My s reg Citroen xsara 1.9 diesel when you turn the key on the speedo jumps to 20 mph and when driving it stays at 20mph plus what it should be I've tried the sensor that is fine clean and with it disconnected the reading is still the same. also tried putting a new earth on no change so any help would be help full. I don't really want to take dashboard out as i believe that is a major headache and i'm not sure where to start in doing that job.