I have an older Canon clc 900 Error E020 Switch

Error :
Intermittent E020 / E061 codes.

Workaround :
It's possible that there is a poor drum ground from the static eliminator brush. You may need to replace the static brush with the drum grounding leaf spring from a CLC 350 (part numberFA5-9308-000). Follow the procedure in technical publication CTP.NO 700/800-69.
Error :
E020 sub code 0440 and light density on all colors

Workaround :
Check the developer assembly movement, i.e. up & down and that the magnetic roller is turning. Check the EPC readings, laser output, fold back mirror, and the connectors for the developer and the developer bias contacts. If all the above checks are good, then the problem can be with the AC voltage output of the developer bias HVT. The DC-voltage for the developer bias is split between two colors ( magenta & yellow ) or ( cyan & black ). The AC voltage is supplied for all four colors and it's loss will be seen in all the colors whereas the loss of the DC voltage would affect only two colors. To fix the problem replace the developer bias HVT part # FG5-3068-060.