I own a Z4, 2 yrs old and only 6,000 miles. It needs a new catalytic converter and luckily the warranty will replace it but I realise that I've probably contributed to its demise by doing mainly short runs and presuming, obviously erroneously, that because I've run the car long enough to get the engine temperature reading to normal that this is enough to clear the cat of any deposits or gunk. In the same way that you advise running the aircon every week, what do you recommend in terms of giving the car a blast every week to keep the cat in good shape and is this based on continuous speed or on a certain length of time?