Code P0446, EVAP System?

Hello, did you get this code after replacing the purge solenoid and I assume the other was the Evap vent solenoid? This code is setting because the conditions in the fuel system were correct for the engine computer to run a self-test, so fuel level, pressure, etc., was correct. And it failed the test. During this self-test the purge valve is opened and the vent solenoid in the back of the vehicle is closed. This allows for the vacuum from the Intake manifold to pull the gas tank into a vacuum to test for any leaks. It then monitors the fuel tank pressure sensor for the rate of vacuum decay. Most likely you have a bad solenoid vent in the rear of the vehicle. What brand purge solenoid and vent valve did you put on the vehicle? If they were aftermarket parts, you may want to get them from the manufacturer (OEM).

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Excessive vacuum would indicate that the system possibly ran its self-test but during the decay time the purge valve was still maybe partially open and still pulling a vacuum on the gas tank, or the fuel tank pressure sensor is reading incorrectly. If you're able to look at Mode 6 data in your scan tool, you may find more information. During the vacuum decay test the vent valve is closed (which is normally opened) and the purge valve (normally closed) is opened to pull a vacuum. I would have to check service info more on Pontiac's Evap tests. I know Ford has a four-stage Evap system testing strategy. It will do the vacuum decay, but also do a vapor generation test where after running the warm fuel in the tank will generate a positive pressure, this is done with both valves closed. So, it can fail this test if anything is leaking, gas cap, purge, vent, canister etc. If you can watch the fuel tank pressure sensor data on your scan tool, pressure should rise slightly after a shut off due to a hot fuel rail. The Fuel tank pressure sensor measures in H20 so it is very precise. The purge valve should be fully closed when the vehicle is off or unplugged. So, verify that it is, you may have to use a hand vacuum pump to check it. But I'll look into the service info a bit more and see what the criteria is for excessive vacuum. Here are some probable causes for that code P0446. Looking at the code, it is a vent control circuit fault. So, if you can bidirectionally control the vent solenoid and also check it with a test light in place of the vent solenoid that would verify wiring. Corroded wiring can also cause faulty readings, because of its adding resistance into the circuit.
Here is some service info on the Evap system self-tests and components.
Another thing that can happen is if the Evap Canister fails, sometimes charcoal pieces can end up in the Purge valve due to the vacuum its pulling under normal purge. If you lightly tap the purge valve and find small pieces of charcoal coming out, they are from the canister, and it has failed being flooded with gas. The small pieces will keep the purge valve from fully closing.

So, the last diagram is the code criteria for P0446, and it sounds a little confusing at first, but it's really not. On this test for the vent valve the ECM is commanding the purge valve closed and attempts to open the vent valve. Its sets the code when vacuum is detected because the vent valve did not open and the fuel system is still pulling fuel from the tank while running, therefore increasing the vacuum, If the vent valve opens like it should, that vacuum will not happen because the tank will be open to atmosphere. So, the excessive vacuum message makes sense.
We would test this by putting a test light in place of the vent valve and commanding it on with a scan tool, The test light should light up brightly when commanded on. And it makes sense that the code is set in the morning, because most systems have to sit for 6 to 8 hours before running the next test.