Most power supply problems will be present even without an output module installed, so check for proper rail voltages that way first.
Could also be a problem on the preamp board, but that usually only happens after a major fault in an output module so comparison-check all the semiconductors between the working and suspect modules with a meter set to diode-check before doing a lot of module swapping.
Problems on anything other than the output modules are kind of a PITA to run down on the CA series due to physical construction (as I'm sure you have noticed).
Probably not the immediate problem - but look for any bulging or leaking main filter caps on the power supply board. Most of the CA amps I saw in recent years had caps that were starting to go. The smaller electrolytic caps on that board also tend to "dry-out" and go high-ESR.