If the copies look fine it is the toner hopper. It does not remove the seal from the bottle when closed. If the copies are getting light it could be the developer unit has worn out bushings and gears thus it does not turn properly making it impossible to sense the toner level and the constant add toner message. Usually you can hear noise if this is the case. Look at the oriface where the toner drops into the photoconductor unit from the toner unit. Is it full of toner coming out the top? If so, time to replace the PCU.