So I got a printer guy, he gave me the following fix and it worked after I too tried everything on the web and forums i could find. It worked first try.
Step 1: Power down the printer.
Step 2: Pull all of your toners
Step 3: Pull your waste cartridge
Step 4: Use the cleaning wand to clean all of your transfer units heads.
Step 5 Optional: Pull each transfer unit one at a time and clean with canned air. Re insert each transfer unit.
Step 6: replace waste cartridge and close door.
Step 7: Power up your C405 (DO NOT REINSERT TONERS YET)
Step 8: After power up, reinsert each Toner one at time In the order the machine asks AND close the door each time.
The idea here is that you clean the toners and transfer units as best as you can and reinsert each toner in a way the machine likes. This worked for me, hope it works for you.
After another hour of troubleshooting I gave up and factory reset it - problem seems to be solved but the reds are very faded.
The dreaded error code has reappeared on our printer
"Reinsert the Magenta toner and reset the device"
I've looked at a number of other threads with the same issue but none of the solutions in those threads or the help article are working
So I'm completely stumped.