It is on the transmission and while it allows the engine to start in P or N it also serves as the back-up lamp switch. For C3 and A4LD transmissions start with disconnecting the negative battery terminal and then safely lifting the car. Pull the harness connector from the neutral start switch, which is located at the bottom right; wipe the area round the switch; then, using a thin-wall socket, remove the switch along with the O-ring. To replacement, obtain a new O-ring and place it on the switch, replace it, connect the harness connector, lift the car and reconnect the negative cable. Check the switch operation with parking brake ON and parked condition, the starter will crank the engine only if the selector lever is in neutral or park position and the back up lamps glow only in reverse gear for C4 transmission for C5/car refer same initial checks as above by removing the negative terminal of battery and lifting the car. Taken out the downshift linkage rod, applied Rust penetrant to the outer lever attaching nut and eliminated the nut and lever. Next, the two retaining bolts for the neutral start switch should be unplugged and to disconnect the multiple wire connector as well, before taking out the switch. First, mount a new switch on the shift transmission, fasten it with the help of the retain bolts, position the switch if necessary, re-install the outer downshift lever and nut and connect the linkage rod using the retain clips. Last but not the least, solder and join the wire multiple connector and check that the engine start only with transmission selector lever in N or P.The following is the procedure for adjustment of neutral safety switch in C5 transmission; Loosen the two switch attaching bolts, place the transmission selector lever in the neutral position, align the switch gauge pin with the ball. The one with black insect, insert the key and go for a road test to ensure the engine starts only in N or P then turns off the switch and tighten the bolts.