Possible Causes

WARNING: This page is about a different car, the 2003 Mazda Tribute. However, it is still accessible from the selected car via links, so may be relevant.

Possible causes are:

Blockages or kinks in EVAP canister tube or EVAP canister purge outlet tube (between fuel tank, EVAP canister purge valve and EVAP canister).
Canister Vent (CV) solenoid stuck open (partially or fully).
Contaminated fuel vapor elbow on EVAP canister.
Damaged Fuel Tank Pressure (FTP) sensor.
EVAP canister purge valve stuck open.
Fuel filler cap stuck closed (no vacuum relief).
Plugged CV solenoid filter.
Restricted EVAP canister.
VREF circuit open (harness near FTP sensor, FTP sensor or PCM).