Ink jet printing apparatus

An ink jet printing apparatus is provided comprising a print cartridge including a heater chip and a nozzle plate coupled to the heater chip. The heater chip has a plurality of heating elements, and the nozzle plate has a plurality of primary and secondary nozzles. Each of the nozzles has one of the heating elements associated therewith for generating energy to discharge ink therefrom. A driver circuit, electrically coupled to the print cartridge, applies firing pulses to the heating elements. The apparatus further includes a nozzle testing station (410) where each nozzle is tested to determine if it is operable. When a heating element associted with one of the nozzles is fired, ink passing from the fired nozzle interupts a beam of light 600a passing from light source 600 to photocell 602, and an ink-passed signal is passed to microprocessor 310. When one of a pair of primary and secondary nozzles is found to be defective, the other is operated in its place.