Over the last few months I have had worn tyres changed and then sometime after discovered what I thought was a slow puncture. I visited a well known chain of tyre fitting centres who discovered that the tyre sensor itself was leaking. the outer sleeve to the valve core has a rubber washer at its base and this washer was leaking air at a slow rate. It turned out that in fact two of the tyres were leaking air at the same time from around the base of the valve. The fitting centre were good enough to remove the leaking sensors on both wheels and fit ordinary valves as a temp measure and at at no cost!! I returned the car to the garage where it was still under warranty to be told it was not a warranty problem and that I would have to pay to have the washers replaced and the electronic sensors replaced in the wheel. I argued this and the service manager reluctantly agreed to do the work at their expense as a 'gesture of good faith'. He had put the fault at my door saying the garage who fitted the new tyres should have replaced the washer on refitting the wheel, there are no instructions in the handbook about this and it is unlikely that this is the case. I have queried this with tyre fitting centres who have no knowledge of having to refit washers each time a tyre is removed from the rim and in fact there is no need for the tyre fitter to disturb the sensor as was suggested by my dealer. Tyre fitting centres do not carry stocks of washers for these electronic sensors and no one knows if it is a standard part to all manufacturers. I have now had the work done and still have not received any official notification with regard to this problem. Be warned and if you have a tyre removed from the rim check with the fitter that this outer sleeve has not become loose and that the washer is in good condition and not split as in my case. I wonder what will happen the next time I approach my dealer with this problem.