Greetings and Welcome to Just answer.
Do you drive lots of gravel?
This code often sets because the vent does not open. I suggest you check that the system is breathing properly. A plugged EVAP canister will cause this.
Let me ask you, does it take fuel slowly
OK, . You need to check the canister for being plugged, and it that is ok, then you need to have the system smoke tested.
If it is plugged, then replace it.
A smoke test is what we use to check for leaks and flow by injecting nitrogen smoke to see leaks and flow.
HOW CAN ONE TELL WHETHER THE CANISTER IS CLOGGED
A smoke test. The system is not letting air through the vent valve to purge the air out of the canister and it sets that code.
ONE MORE THING: THE CATALYST ON THE TRUCK WOULD NOT GET READY EVEN AFTER DRIVING 1500 MILES. IS THERE A RELATIONSHIP TO THE EVAP PROBLEM.
GM DEALER HAS FLASHED THE PCM WITH THE MOST UPDATED PRGRAM 3 WEEKS AGO.
Yes, there likely is.
OK, if the evap is not ready, and a code is set such as the P0446, then it will not pass.
It will not fully run if there is a fault in the EVAP system.
IT depends on the software and PCM. These are a bit of a different beast. I would repair the P0446 and retest.