Code P0446, yet I/M status reads "Evap System OK"

Question

I am getting the P0446 code on my 2001 GMC van. My reader says the code indicates a problem with the Evaporative Emission Control Circuit. Just based on that, i have changed the gas cap, the vent valve, the purge valve and the vapor canister - no change. Finally i discovered that when i look at the I/M status on the reader, all monitors are ready, the EVAP System has an OK, passed the test, all other monitors are OK except 2, the "2nd Air System" is NO, and the "O2S Heater" is NO. These 2 monitors that did not pass are not related to the Evaporative Control circuit- is it possible that the code P0446 is set in error by the computer ? Has anyone encountered something similar ? Thanks for any help or advise