You'll want to find out how well your Toyota Corolla did on the rest of the emissions as well; CO and NOx. HC is basically raw fuel (gasoline). High HC means raw fuel is leaving your Corolla's tailpipe when it should be burned in the engine.
It is important to know if your Corolla is in proper fuel control. Air/fuel mixture must be 14.7:1 in order for fuel to burn efficiently. By looking at the CO emissions which your car produced during the smog test we can ascertain approximately where the fault is that's causing the high HC failure.
If CO emissions where also high during the smog test, it is very possible that your Corolla is producing high HC due to a rich fuel mixture. If CO is low, then the opposite applies; the air/fuel mixture might be too lean.
We have to recommend you have a smog check repair center before a Fuel Feedback Test. This test will ensure your Toyota Corolla is in proper fuel control, and that air/fuel ratio is as close to 14.7:1 as possible. During the feedback test, a smog technician will check all emission components which are responsible for sensing air intake, oxygen levels in the exhaust, and fuel delivery, as well as whether your Corolla's computer is receiving and interpreting sensor data properly.
For high HC faults not related to fuel control, the most typical faults have to do with improper spark delivery caused by a defective or damaged ignition system components. We are referring to your Toyota Corolla's spark ignition system; not to be confused with the key ignition system.
As mentioned above, while diagnosing high HC faults it is important to know how much CO the vehicle produced during the smog check as well. A byproduct of incomplete combustion is CO (partially burned fuel). High CO along with high HC indicates a rich mixture problem, while low CO and high HC indicates a misfire condition due to an inadequate amount of fuel entering the combustion chamber. In both scenarios, whether high or low CO was produced, a byproduct of the inefficient combustion will be high HC.
