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To test the Honda CX500 CDI box you will need a multimeter. More complex testing can be done with an oscilloscope but few people have access to those...
Under your seat you will find a connector with a blue/white wire, coming from the stator and going to your CDI box:
You want to test the side which is coming from the stator, not the side leading into the CDI box. Set your multimeter to 2k ohms resistance, and measure the resistance between the blue wire to the white wire, followed by the white wire to ground.
If that is not within spec head over to the stator test page and complete that as you may have a problem with your stator. Assuming those numbers are OK, reconnect the blue/white connector, and find the yellow/white/black-with-white-stripe wire coming from the CDI that leads towards the front of the bike;
Set your multimeter to read DC Volts with a range of 250. Clamp your black multimeter probe to ground, then test the yellow and pink wires in turn by pressing your probe into the end of the wire whilst cranking the bike for a few seconds. You won't get a steady reading, but you should see at least 150V DC flicker on the box if your CDI box is working properly.
If both sides check out OK, test again with the black-with-white-stripe wire connected and the killswitch in the "run" position. If you don't get the same results the killswitch is probably shorting to ground.
If you are having trouble with the bike not stopping when you hit the killswitch, you can try disconnecting the black-wire-with-white-stripe while the bike is running and grounding it; if the bike does not stop immediately it's very likely that you have a faulty CDI box.
Credit to Reg from Bristol for the original writeup of this procedure.
If your CDI unit is broken you may wish to head over to the "Alternatives to the CDI box" section of this page.
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