Hi Peter, I think that you may of partially answered your problem already. If the vehicle is running fine until it gets up to operating temperature, then I think you may have a temperature related problem??? Usually on the vag range of vehicles, the CTS is prone to failure and will sometimes even set a value of -40 degrees when going out of range or open circuit. Take a look at the live data and see what the CTS is reading when the fault occurs. HTH Pete & good luck, Gunner.