Hi,
If that solenoid isn't responding when tested, that is the likely cause. When the solenoid is off, the vacuum signal from the intake manifold is cut. When the ECM sends an ON signal the coil pulls the plunger downward and feeds the vacuum signal to the power valve actuator.
The P0300 is a result of the condition, and the P1456 would not cause what you described.
So, if you already confirmed it isn't responding, I would suggest replacing it. If you would rather confirm it through the diagnostics, I attached them below for you.
Let me know what you find and if a new solenoid takes care of the issue.
Take care,
Joe
See pics below.
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Thursday, December 8th, 2022 AT 8:46 PM