Not necessarily, they run about 100 bucks each. Why spend 300 when you only need to spend 100.
On the CDM modules CDM #1 gets its charging ground path through CDM #2 or #3
CDM #2 and #3 get their charging ground path through CDM #1
A shorted Stop Diode in CDM #1 would prevent CDMs #2 and #3 from sparking.
A shorted Stop Diode in CDM #2 or #3 would prevent CDM #1 from sparking
You have a couple of different ways ot go about this. You can swap 1 and 3 and see if spark returrns on #2 or not. If it does, CDM 1 is actually bad, not sending a ground signal to CDM 2. Or swap 2 and 3 and see if 2 starts sparking again, and if 3 dies, then it is a bad CDM #2. Or, just get 1 new CDM and stick it in #2 spot and see if it sparks. If not, put #2 back and then put the new on in the #3 spot.
It's a funny system and I know that sounds complicated. But does that hopefully make any sense to you?