Pulled two codes for the EVAP...need help

[mat_man1]

I was at Advance Auto today and had the guy run the diagnostic test on my 97 hardbody. CEL has been on since I got it about 10000 miles ago. Had it diagnosed before but the last associate wasn't too knowledgeable and could not really tell me what it was. Runs fine as far as I can tell but it gave two codes: P0450 EVAP Emission Control System Pres Sensor Fault and P0446 EVAP Emission Control System Vent Circuit Malf. All both employees could tell me is that the gas cap might not be tight enough or I may need a new gas cap. Is there something else that could trigger these codes. I know not tightening the cap can cause the ses light to come on but I wouldn't think it would set off TWO codes. Also is the ECU on the 97 Pickup set up like my 98 maxima where I can run my own codes by the ses light flashing and if so where is it because it is not in the same location and I can't find it. Thanks.