When you have a 50 error on the 4/4+/5 machines, you have power off for 15 min after installing the new fuser until it clears the error. If the error doesnt go away, replace the power supply RG5-0531. You probably can buy it from Printer techs or PM me and I'll sell you one. Better yet upgrade to a newer printer, cause HP 4's are old. Your decision.