Your mechanic has already checked for the main problems which cause high HC, which are ignition related, i.e. spark plugs, wires, distributor and cap and rotor. Have they checked engine compression? If engine ignition is good and the air/fuel mixture is correct, we recommend ensuring engine compression is normal also. Without proper compression your Toyota Camry's engine will not ignite the air/fuel mixture properly.
Next, it is very important to diagnose CO emissions. If your engine is producing low CO (along with high HC) we'd consider looking into faults which would cause lean fuel mixtures. If CO is high (accompanied by high HC) we'd consider faults which would cause a rich fuel mixture.
The oxygen sensor, along with various engine emission sensors, are responsible for allowing the engine control computer to deliver the correct amount of fuel to the amount of air entering the intake manifold.
A thorough diagnosis should reveal the fault(s) which are causing your Toyota Camry to produce high HC. Diagnosing CO levels is an important part of the overall repair process.
Furthermore, we recommend only using the gas octane level required for your vehicle which should be 87 Octane. Using a higher octane only assists in situations where NO levels are high, and will usually lower NO but raise HC and CO. Only use the recommend octane level for your vehicle.
As far as foul play... on an older model vehicle which has failed the smog test we can not suspect foul play. It is very common for a vehicle with high mileage and/or wear and tear, to develop problems which will need to smog test failure.
To learn more about high emissions and smog check failures please visit this page.
