All three emissions the smog inspection tests for (HC, CO and NOx) are interrelated. One's results can effect the others. It is important to know what the CO results for your Geo Prizm were as well, in order to determine what the cause of the high HC fault is.
The main reason your Geo Prizm may have been producing high HC could have been due to the ignition system, but since you mention your car was recently tuned-up and still failed (with even higher HC), we have to assume there are other faults. We recommend having a certified smog check repair center conduct a hands-on smog check failure diagnosis. Follow the link for more information on High HC.
