Epson R320 system error. ?

I have tried all the usual cures. Also dismantled and reassembled printer. One thing always brings it back to life. I remove all cartridges. Printer gives can't recognise cartridges message. Press maintenance, put in cartridges. Perhaps one or two cycles of unrecognised for one or two colours, then all in order, goes through charging cycle, and works perfectly. But if I switch off and on again, back to square 1 (system error).
By confusing the printer (no cartridges) I must be by-passing some check the printer normally performs and fails at. But what is it, and has anyone managed to put it right?

Andrew Dunstan
April 2018