http://www.240sx.org has detailed isntructions on cdiagnosing your engine light and reseting it....its pretty simple you can do it yourself the only hard part is removing the ECU (I have a 96, and removing it is tricky)..... shops usually diagnos your engine light for about $70....
also if you're SURE that it has something to do w/ the 300Z filter and you don't want to spend $ at a shop just simply remove your battery and *touch the red and black wires (it might cause a little spark...) and leave the battery wires off for at least 12 hrs (overnight) and this will "resets" your ECU because it causes the ECU to lose its memory
*I don't know if touching them together is necessary but it is what I've been told...you can try it w/o touching the 2 ends