There are two things you need to know when checking for an EVAP leak: where the leak is and how big it is. The most common place for an EVAP leak is at the connection between the fuel tank and the charcoal canister. This is because this connection is constantly exposed to fuel vapors, which can cause corrosion over time. The other common place for an EVAP leak is at the purge valve, which is located on the engine. This valve allows air to enter the fuel system so that the fuel can be burned more efficiently. If this valve becomes corroded or damaged, it can cause an EVAP leak.