You will first need to find out if your 1998 Toyota Camry's fuel feedback system is working properly. This includes ensuring your Toyota's oxygen sensor is working properly, your Toyota's emissions computer is receiving the signals from the oxygen sensor, the computer is computing the data properly and sending the correct signals to the fuel injectors to either increase or decrease fuel delivery to the combustion chambers.
A smog station would conduct what is referred to as a "feed back test". They will utilize a 5-gas analyzer (usually the smog machine) and propane. The smog technician should introduce propane to the intake system and ensure the oxygen sensor is reading this introduction as an increase in CO, and ordering the computer to decrease fuel delivery at the instant propane is added to the system.
At the end of the feedback test it will be know whether the high HC fault is electronic or mechanical, and the technician can further diagnose the fault.
Hydrocarbon (HC) is basically raw fuel, otherwise known as gasoline. Typically vehicles fail for high HC due to defective spark delivery, however there is a condition called the "Rich Misfire" (or Lean Misfire) which your Toyota Corolla might be suffering from as well. This condition is created, when for any reason, the combustion chambers receive too much fuel (or not enough fuel; the fault is a Lean misfire) causing the fuel that is delivered to burn improperly and leave behind high levels of raw fuel(HC) and partially burned fuel(CO). Vehicles producing high HC for fuel management reasons usually begin producing normal HC levels once the fuel problems are corrected.
However for those vehicles suffering from high HC not related to fuel management, should have the engine ignition system closely inspected.
