My Renault Megane has a tyre pressure monitoring system fitted. Recently it alerted me that one tyre had low pressure. I drove to the garage to put some air in. However when I removed the plastic valve cap I was very shocked to see sections of the threaded valve literally crumble into small pieces and expose the inner valve stem. I drove round the corner to my local very friendly tyre fitting place, who agreed with me it was dangerous and he then removed the tyre, fitted a standard valve and replaced the tyre, free of charge (that's service for you). It turns out the valve is made of aluminium and despite having valve caps on in line with the manufactures guidelines, plastic not metal, they still corrode. I hate to imagine what would have happened if it had disintegrated at speed. Given the obvious safety issues, plus the actual cost of replacing and reprogramming these sensors to the car (£140 a time) I feel this needs to be brought to owners attention.