2000 Yamaha YFM400FWA(M) Factory Service workshop Manual, Page 349Get this manual
SIGNAL SYSTEM
5.If the coolant temperature indicator light does not come on when the start switch is pushed on, or if the coolant temperature indicator light does not come on when the temperature is high (more than 107113 (224.6235.4 F)): 1.Bulb and bulb socket
NO CONTINUITY
Check the bulb and bulb socket for continuityCONTINUITY Replace the bulb andor bulb socket
2.Thermo switch