The most common fault, hands down,  is a fault with the “EVAP” or Evaporative Emissions System on any vehicle.
Having a Technician perform a diagnostic and “scan”  your system for the “code” or “fault code”  is only the first step in many that is required to diagnose the problem you are having with your evaporative emissions system.  Codes that are common for EVAP are P0440, P0441, P0442, P0443, P0445, P0446, P0450, P0451, P0452, etc.
A general overview of the purpose and function of the EVAP system is as follows:
In order to prevent the leak of fuel vapors (raw gas also known as hyrdocarbons) into the atmosphere (hyrdrocarbons become a greenhouse gas), your vehicle is designed with a fuel tank system that is sealed from leaks.  In order to accomplish this, the fuel vapors from the tank must be removed and burnt with the gasoline (otherwise on hot days the fuel tank would expand and then contract on cold days).
This process involves: intermittently opening a Purge Valve that allows the engine to draw fumes from the tank.  At the same time, the Vent Valve opens and allows fresh air into the fuel tank (so that the tank does not contract/implode).
In order to ensure that this system is functioning properly, your vehicle’s on-board computer will intermittently run a test on the system as follows:
1. Open Purge Valve while keeping the Vent Vale CLOSED, thus creating a vacuum or lower than atmospheric pressure in the tank.
2. The Purge Valve is then closed, trapping the low pressure condition in the tank.
3.  The Vapor Pressure withing the fuel tank is monitored, over time, to ensure there are no leaks.
4.  Once the test has been performed, the “Readiness Monitor” for EVAP will become “Ready”.
Within the fuel tank there is a vapor pressure sensor that monitors the vapor pressure (similar to a barometer used for weather) or actual gas pressure of the vapors in the tank.  In order to pass the test, the low pressure trapped within the fuel tank must be maintained steadily throughout the test period.  If there is a leak anywhere in the system, fresh air from outside the tank will be sucked in and the vacuum will be reduced or lost, returning the tank pressure closer or equal to atmospheric.  Also, if any component fails to operate properly (not closing or opening as commanded), the vacuum will either not be created or will be lost, causing a failure of the test.   After 2 consecutive failures of this on-board test, the Check Engine Light will be illuminated and a fault code will be stored within your car’s computer system.
The electrical and wiring connected to the valves and sensors of the EVAP system are also monitored by your vehicle’s on-board diagnostic system.  Should a “short” or “open” take place within one of the related circuits, the Check Engine Light will become illuminated, storing the appropriate code in the on-board computer.
A leak in your EVAP system could be caused by:
Leaking fuel tank
Leaking/inoperative vent valve
Leaking/inoperative purge valve
Inoperative/defective fuel tank vapor pressure sensor
Leaking vapor/vent lines to/from from tank/charcoal canister/engine
Leaking charcoal canister
Leaking fuel filler neck
Leaking/loose fuel cap
Wiring to/from vent valve/purge valve/fuel tank vapor pressure sensor/computer
Basically, the procedure for testing this is to function test all of the valves and components and if they are working, test for leaks.  Testing for leaks often involves the use of a smoke machine to pressurize the fuel tank and system with smoke. Any visible smoke seen coming out of the system usually signifies a leak.  We also use soapy water find to find leaks if smoking it does not give results.
Depending on the fault, it could take anywhere from 30 minutes to 3 hours to find the problem.  In some cases, the leak can be at or near the top of the fuel tank, making it necessary to lower or even remove the tank.
Please contact Car Medics for further information so you can get it fixed right, the first time.