having just had similar oil issues on my yam I spent many hrs sorting various and buying new parts not required
for it to turn into a real simple fix, bad ground, see below,
According to the manual the yellow light is triggered by the oil sensor in the remote tank. The red light is triggered by the oil sensor in the engine tank
The sensor switch in the remote tank is normally closed when the float is up (oil in the tank) and provides a ground to the CDI/ECU/Oil module and the ground to the tachometer for the warning light. If and when the float drops the switch opens and removes a ground.
The switch is apparently working and telling the CDI that there is sufficient oil in the remote tank for auto transfer to occur. But, maybe the ground from the switch is not getting to the tachometer, in which case the yellow light will illuminate.
Check the connections in the trim/oil harness at the tachometer. Check the connections to the trim/oil harness in the engine pan.