The code: P0446 - Evaporative Emission Control System Vent Control Malfunction.
This is not related to the brake booster.
The poor fuel mileage issue is likely caused by the EVAP system malfunction.
This code can be triggered by few different reasons. It could be the vent valve, it could be the fuel filler neck, it could be the harness to the valve etc.
I have included the complete troubleshooting procedure below. Go thru this to find out the issue.
Please let me know of any questions.
Images (Click to enlarge)
Friday, October 1st, 2021 AT 7:45 PM