For about the past six weeks the car has been setting a P0443 code, which is a problem with the EVAP canister purge solenoid control circuit. I thought it was the solenoid, but then whenever I'd fill the gas tank, the light would go out. The SES would come back on when the car would use about 8 gallons of gas, then go off again when it was filled, then the light on light off cycle continues. Use about 8 gallons, light on, fill it up, light off. I also noticed I no longer hear the psst sound when the gas cap is removed, so am wondering if the cap is no longer sealing properly, and when the tank is full it negates the effect of the cap not sealing properly. Cap or solenoid?

 

here's some copy\paste on trouble shooting.

DTC P0443: EVAP PURGE SOLENOID CONTROL CIRCUIT (3.8L "C", "F", "G" & "W" BODIES) Circuit Description Ignition voltage is supplied directly to EVAP purge solenoid valve. PCM controls valve by grounding control circuit via an internal driver. Primary function of driver is to supply ground for component being controlled. Each driver has a fault line which is monitored by PCM. When PCM is commanding a component on, voltage of control circuit should be low (near zero volt). When PCM is commanding control circuit to component off, voltage should be high (near battery voltage). If fault detection circuit senses a voltage other than what is expected, this DTC will set. PCM will monitor EVAP purge valve circuit for short to ground, short to voltage, open circuit, open component or an internally shorted or excessively low resistance on EVAP purge valve circuit. When PCM detects any of these malfunctions, this DTC will set and affected driver will be disabled. PCM will continue to test circuit and if fault disappears, driver will be reactivated. This DTC will set when there is short to ground, open, or short to battery voltage detected on control circuit and the condition is present for at least 30 seconds. Diagnostic Procedures 1. Perform On-Board Diagnostic (OBD) system check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS INTRODUCTION article. After performing OBD system check, go to next step. 2. Turn ignition on, with engine off. Using scan tool, increase and decrease purge solenoid up and down. If solenoid responds as commanded, see DIAGNOSTIC AIDS . If solenoid does not respond as commanded, go to next step. 3. Turn ignition off. Disconnect purge solenoid. Turn ignition on, with engine off. Using a test light connected to ground, probe purge solenoid ignition feed circuit. If test light illuminates, go to next step. If test light does not illuminate, go to step 10 . 4. Connect test light between purge solenoid ignition feed and ground circuits. Using scan tool, command purge solenoid on and off. If test light turns on and off, go to step 8 . If test light does not turn on and off, go to next step. 5. If test light remains illuminated, go to step 7 . If test light remains off, go to next step. 6. Check purge solenoid for an open or short to voltage. Repair as necessary. After repairs, go to step 13 . If circuit is okay, go to step 9 . 7. Check purge solenoid control circuit for a short to ground. Repair as necessary. After repairs, go to step 13 . If circuit is okay, go to step 9 . 8. Check for faulty purge solenoid connections. Repair as necessary. After repairs, go to step 13 . If connections are okay, go to step 11 . 9. Check for faulty PCM connections. Repair as necessary. After repairs, go to step 13 . If connections are okay, go to step 12 . NOTE: For circuit reference, see appropriate wiring diagram in WIRING DIAGRAMS article.
10. Repair purge solenoid ignition feed circuit. After repairs, go to step 13 . 11. Replace purge solenoid. After repairs, go to step 13 . 12. Replace PCM. Program replacement PCM using required equipment. After repairs, go to next step. 13. Using scan tool, clear DTCs. Operate vehicle under conditions required to set DTC. Check for DTCs. If DTC P0443 is set, go to step 2 . If no DTCs are set, system is okay. Diagnostic Aids For any test that requires probing PCM or component harness connectors, use Connector Test Adapter Kit (J- 35616-A). This will prevent damage to harness connector terminals. Check for poor connections at PCM or at component. Inspect harness connectors for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals and poor terminal-to-wire connection. Check for misrouted wiring harness. Inspect harness to ensure that it is not routed too close to high voltage wires (spark plug wires) or too close to high current devices (generator, motors, solenoids). Check for damaged wiring harness. If harness appears to be okay, observe scan tool while moving related connectors and wiring harnesses. A change in scan tool display may help to locate fault.