2004 Official factory service manual for Yamaha YFZ450S ATV Quad., Page 333Get this manual
LIGHTING SYSTEM 2The tailbrake light fails to come on1Tailbrake light bulb and socket Check the tailbrake light bulb and socket for continuityRefer to "CHECKING THE BULBS AND BULB SOCKETS" Are the tailbrake light bulb and socket OK? YES NO Replace the tail brake light bulb, socket or both2Voltage Connect the pocket tester (AC 20 V) to the tailbrake light coupler as shownTailbrake light coupler (wire harness side) Tailbrake light Positive tester probe blue Negative tester probe black 2