Found this after a little research, I know for a fact that it cannot be the gas cap. I have been driving the car now and everytime I fill up, the engine is off.... I do notice a little vacuum when opening the gas cap though (hmmmm) I guess it is time to bench test this sucker.
An OBDII car also monitors and tests the emissions system through the ECU. P0443 and P0446 are codes corresponding to the EVAP system. The EVAP system uses a charcoal canister to capture raw fuel vapors (hydrocarbons) from the fuel tank so they aren’t released into the atmosphere. The charcoal adsorbs hydrocarbon vapor, meaning the vapors are held on the surface of the charcoal and can be released by a burst of fresh air. When the ECU determines it should purge the canister, a vacuum is created that pulls fresh air through the carbon canister to the intake manifold. The valve opens and the vapors are consumed by the engine. The EVAP system is a non continuous OBDII monitor, only active certain times. A sensor in the EVAP system monitors fuel tank pressure to detect restrictions and fuel vapor leaks of no more than 0.04” in diameter in a hose or filler cap. These leaks can be caused by something as minor as a gas cap that hasn’t been tightened enough. However, Nissan issued several technical service bulletins regarding the EVAP systems on 1996 and 1997 Maximas. P0443 is the code for the Evaporative Purge Volume Control Solenoid Valve and P0446 is the EVAP Canister Vent Control Valve (Circuit). They control fuel tank venting air flow volume to prevent hydrocarbon's from escaping the fuel system and polluting the atmosphere. A properly functioning EVAP system consists of two functions- detecting leaks in vent lines and the hydrocarbon airflow purge from the canister to the engine. Your system is not properly purging the canister. Caused by water entering the hoses and corrosion and, believe it or not, spiders building nests in the vent tube, failure to the canister purge solenoid is common in these vehicles. The system was later updated to include a water separator which should be installed when replacing the solenoid to prevent further defects to the EVAP system. If the canister itself is not working properly, charcoal pieces may have obstructed the solenoid. There are 10 pieces to the package and not all of them may need to be replaced. A flow test of the canister purge valve and a pressure test of the EVAP system with the engine off can be performed to determine leaks in the fuel tank, lines, or cap and fix the problem. Although the EVAP codes don’t cause any performance issues, you will need to fix them if you want to pass emissions. While some OBDII codes only remain on until the system that failed passes a self-test on 3 consecutive trips, emissions problems need to be fixed before the code can be erased.
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