Your car's evaporative emissions system tests itself with engine vacuum.
There's a valve on the charcoal canister which closes and a purge solenoid
on the engine which cycles to vacuum the system. The code P0446 typically
means that there's too much vacuum in the system during non-test times.
This usually means that one of the following has occurred:
The close valve (the one on the canister) stuck closed, causing the system
to not be able to vent properly.
The purge valve stuck open, causing constant purge (and more vacuum than
There is a restriction in the venting of the canister. This could be in
the canister itself, in the vent filter, or a clogged hose.
The fuel tank pressure sensor was inaccurate, causing the computer to be
fooled into thinking there's too much vacuum in the system.
I've seen all of the above. What's worse, running the evap test may not
make the code reoccur. I'm surprised tech assistance advised him to
diagnose the problem in this fashion.
Basically, what he did, was check the system the way they requested as
well as check some of the connectors to verify good contact, and didn't
find any problems.
If there's a problem still there, you check engine lamp will likely come
back on within the week.