Hi, Howard for your neutral light the usual suspects are:
1. Faulty neutral switch.
2. Grounded neutral light circuit.
If you have a dash, instrument, turn signal, tail lights, and speedometer on some bikes go out at the same time and you have confirmed that the bulbs are good then you need to do some quick diagnostics all you need is a test light and a wiring diagram usually found in the back of your service or owners manual.
1. Make sure your battery is fully charged with "CLEAN" and "TIGHT" connections and the instrument light fuse is not blown.
2. Inspect the bulb socket for corrosion and clean as necessary, a spray can of electrical contact cleaner and compressed air works the best.
3. Confirm your test light is working properly by connecting one end to the battery negative post and touching battery positive post with the sharp probe, the test light should illuminate.
4. With the ignition turned on confirm you have power to the bulb socket by connecting your test light to battery negative or ground and probe the center contact, the test light should illuminate if not you need to check the integrity of the switch or sending unit the light represents, if that is ok then you have a short in the wiring between the component and the light bulb, if all is ok proceed to the next step.
5. Confirm you have a good ground to the light bulb socket by connecting the test light to battery positive and probe any metal part of the socket housing, the test light should illuminate, if not you need to find the short and repair as necessary.
6. Inspect the integrity of the spring loaded positive contact insulation tab by pushing against it with your test light to see if it goes down and springs back fully to it's operating position again contact cleaner and compressed air works best to restore proper function.
7. If your bike has a lot of mileage the vibration can cause bulbs to become loose in the socket by wearing down the soldered contact thus loosing spring pressure from the tab and eventually stop making contact this can be fixed by replacing the bulb or putting a drop of solder on the worn out end to ensure contact.
8. If all of your dash lights are out but are good then you most likely have a faulty connection to the dash wiring harness, inspect the connector for corroded, broken, or loose pins/sockets.
For more information about your issue and valuable "FREE" downloads that you will need please click on the links below. Good luck and have a nice day.
Yamaha xt 125 service manual
OEM parts for Yamaha
Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/gregg_c0ec1df182c7330e