You test the incoming volts from the stator and the timer base while everything is hooked up. If it all checks good, then you check the output of the cdi box. If it does not test good voltage out, then the cdi box is bad. When testing, if you have a bad stator or timer reading while hooked up, then recheck them when not hooked up. If they test good, then the cdi box is usually shorting the voltage out and is bad. There are continuity test to do on the stator and trigger (timer base), but not on the cdi box.