An ink jet printing system comprises multiple print cartridges each containing a different ink and having a plurality of printhead nozzle arrays scanning across printing media in a printing zone. After a specified amount of ink has been dispensed, an ink jet printhead carriage is moved to a refill station for ink replenishment. A label on a print head service module displays encoded visual indicia which are sensed by an optical sensor on the ink jet printhead carriage. The optical sensor is used to read pre-encoded information as well as on the fly information which is recorded on a label when the carriage moves along a traverse path to the printhead service station.
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