validation routine of the invention is implemented and the vehicle ignition switched on step 12 other systems may be interrogated to ensure that the vehicle is ready to move step 13 and confirmation is obtained that the engine is running step 14 csn is assumed non active step 15 and the catalyst temperature is determined step 16 if less than the threshold the routine continues if not csn is disabled step 31 step 16 prevents activation of csn if the catalyst is at or above operating temperature csn is disabled until the next engine start event step 32 at step 17 a diagnostic check ensures relevant communications are working for example control inputs from the braking system and transmission selector a fault disables csn at step 18 the temperature of automatic transmission fluid is checked csn may for example be inhibited step 33 if atf temperature is below a predetermined minimum of less than zero say u2212 6 u00b0 c at step 19 a check is made that the vehicle is stationary for example by signal from a motion sensor if no csn is inhibited at step 20 a check is made of vehicle attitude for example to determine a minimum brake pressure which will hold the vehicle stationary above a threshold csn is inhibited csn may also be inhibited if the vehicle attitude exceeds a predetermined value i e the vehicle is strongly inclined say above 15 u00b0 or above 20 u00b0 fore and aft at step 21 a check is made that brake pressure applied by the driver or a hill hold function is above a minimum to hold the vehicle.