HC (Hydrocarbon) is basically raw fuel. When you are filling gas at a gas station, you are filling HC into the gas tank. High HC during the smog test means raw fuel is leaving your Toyota Camry's tailpipe when it should be being burned in the combustion chamber. Something is causing your Toyota's engine to not burn fuel as efficiently as possible; leaving behind high levels of raw fuel (HC).
The reasons your Toyota might produce high HC (not burn the air/fuel mixture entering the combustion chambers completely) can be more than a few. The most typical fault is due to improper spark delivery caused by a defective or damaged (old) ignition system. We are referring to your Camry's spark ignition system; not to be confused with the key ignition system.
While diagnosing high HC faults it is important to know how much CO your Toyota Camry 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 (more fuel then required entering the combustion chamber), while low CO and high HC indicates a misfire condition due to an inadequate amount of fuel entering the combustion chamber. In both scenarios, high or low CO, HC emissions will be elevated.
If your Toyota Camry failed for high HC but was produced normal CO emissions, we would suspect the vehicle's ignition system (spark) has a fault. We'd check the spark plugs, spark plug wires, distributor and distributor cap and rotor. Air/Fuel ratio must be at an optimum 14.7:1 in order to fuel to burn efficiently.
If CO was either too low or too high, we'd suspect a faulty fuel control system or mechanical fault causing too much fuel to enter the engine. A hands on diagnosis will be required to determine the root cause of your vehicle high HC fault.
The link below will give you a full list of problems which could cause high HC smog check failures.
