It's great that the NOx numbers decreased significantly. Now you have to work on the HC problem. If CO is normal (between 0.00 and 0.07) you should look into your Nissan ignition system... the sparkplugs, wires, distributor cap and so on. High HC faults, if not related to fuel control, are usually caused by worn ignition parts.
If CO is not normal, too high or too low... then you want to look into the fuel control system. High CO (rich fuel mixture) or low CO (lean fuel mixture) will cause high HC as well.
