Mike, a diode is nothing but a check valve. If you have a digital meter it should have a diode function on it to check it. If the wire is broken or the diode is open it will cause the issue you are having. If it is shorted it will cause the mags to kill each other and not run correctly. I had a shorted one on a 3165 and ordered 2 new mags but they didn't ship diodes with the new mags that I ordered so I made a trip to Radio Shack and picked these up. They are rated at 1.5 amps @ 1000 volts and their part number is 276-1114. I think they were about $2 or $3 for 3 of them. They have a mark on one end and that mark goes toward the mag. Here is a link on how to check them also.
http://www.allaboutcircuits.com/vol_3/chpt_3/2.html.