The valve body is the brain of the transmission. Up and down shifting is controlled by the valve body through hydraulic pressure and electronic commands from the vehicle's performance system. The valve body directs the flow of transmission oil to where it is needed in order for the transmission to perform a function (for example, the application of "passing gear"). The environment within which the valve body operates must be sterile. Varnish buildup and wear material can clog the valve body, causing erratic shift patterns of the transmission. This is why the transmission fluid and filter should be changed every 30,000 miles.