The faders banks can be replaced in banks of 8, Yamaha went through several designs of fader on the M7CL and at one point we replaced all the faders on ours and the new ones had much better dust sealing than the ones that were replaced. I can't remember how modular it is inside but a checksum error is indicative of the flash on main board not being happy. Again the board is replaceable although Yamaha may not be able to supply them anymore. We sold our M7 and moved to a CL3 a few years ago. It may still be worth trying to repair your M7 though as replacing it currently may be hard, Yamaha are prioritising parts to higher margin products so the lead times on CL3s are very long as they are sending common parts to make PM3s instead.