There could be a multitude of reasons which would cause your Geo Prizm to produce and fail the smog check for high HC. It's important to know how much CO your Goe Prizm produced as well in order to know better what the cause of the high HC is.
HC (Hydrocarbon) is basically raw fuel. If your Geo failed for high HC but was producing normal CO we would suspect the vehicle's ignition system (spark). HC emissions have a direct correlation to CO. The amount of HC left over after the combustion process is based on the amount of fuel having actually entered the combustion chambers. A "rich" or "lean" fuel mixture would contribute to high HC. Air/Fuel ratio must be at an optimum 14.7:1 in order to fuel to burn efficiently.
We recommend first inspecting your Geo Prizm's ignition system first. Since you have already replaced the spark plugs, inspect the spark plug wires, distributor and coil. If everything looks ok, move on to the inspection of the fuel delivery system, i.e. the oxygen sensor (s), engine coolant temperature sensor, manifold absolute pressure (MAP) sensor. The MAP sensor is vital to proper air/fuel ratio. Your vehicle's ECU relies heavily on the MAP sensor input to determine the amount of fuel to deliver the combustion chambers. NOTE: Inspecting these components will require some auto emissions know-how and proper inspection tools. If you are capable, a repair manual will help.
The link below will give you a full list of problems which could cause high HC smog check failures.
What Causes High HC?
