Small EVAP system leak detected
Probably the most common Evaporative Emission Control System (EVAP) related code on a Toyota, P0442 signifies that a small leak has been detected. Most likely, the engine control module has found detected a fuel vapor leak in the EVAP control system that's so small it's not likely an indication of a major failure. A pinhole leak somewhere before the purge control valve (PCV) that causes a loss of vacuum pressure can be enough to trigger a P0442.
The EVAP system's job is to collect gasoline vapors from the fuel tank and fuel lines. These vapors are stored in a charcoal canister (which is usually under the hood in the engine compartment). When the engine is running, this vapor is bled into the cylinders to be burned off, preventing dangerous (and bad for the environment) gasoline vapor from leaking into the atmosphere.
Nearly every time a P0442 is triggered on a Toyota, it will be because the gas cap is missing, improperly affixed (i.e. not closed tightly), or leaking. This code may also come up when a gas cap is replaced with a non-OEM gas cap that doesn't fit correctly.
Most of the time, removing and putting the gas cap back on and then clearing the code will solve your vehicle's P0442 issue. If that doesn't work, you may need to replace your gas cap with an OEM gas cap (assuming you've got a generic cap installed now).
If the problem is not in the cap, however, it may be a pinhole leak in one of the vapor hoses or tubes that runs from the fuel tank vent to the charcoal canister or one of the seals on that tubing's connections.
Testing the hoses for leaks can be time consuming, so most mechanics will test them at their connection points (the most likely point of failure) to the fuel tank, splices in the tube itself along its run to the engine compartment, and at the connection to the charcoal canister. If no leak is found, the rest of the tubing may be methodically checked by putting it under light pressure and using the soapy water technique. Often, however, replacing the tubes is cheaper and easier than spending this time testing them.