Replaced all 4 O2 sensors in response to Serv. Eng. light and code indication.

Reset computer, started car and car ran for approx 1/2 hour without Ser. Eng light on then light came back on. Had codes checked and two codes gave the following;

1. EVAP canister vent control valve (close) - no purge detected during purge event. Probable cause.
A) Hoses connected incorrectly
B) Failed canister purge solenoid
C) Failed fuel tank pressure sensor
D) Open circuit condition

2. Catalyst system below threshold - bank 1
Probable cause
A) Large vacuum leak
B) Rich AF ratio - fuel system fault
C) Ignition system malfunction
D) Faulty Cat. Conv.

After resetting computer (car sat for 2 hrs), started again and no "Ser. Eng" light... drove for approx 1/2 hr and light came on again with same codes. I would assume delay rules out A) & D) in #1 and A), C) & D) in #2... I was considering replacing the fuel tank pressor sensor but am at a loss, any ideas, suggestions or advice?

What year is the car? From my experience evap codes will set of other codes, so you should fix the evap one first. The cars computer checks for some codes at different times, so clearing the code turns off the light until the next checkpoint. Most evap codes are set off by problems with the evap canister or the purge solenoid that is attached to it. These links provide a lot of info.
Most Nissan products have similar systems.


http://www.automotiveforums.com/vbulletin/showthread.php?t=445827

http://www.asashop.org/autoinc/feb2006/techtips.htm

http://www.nissanclub.com/forums/engine-performance-1993-2001/220611-evap-vent-control-valve.html

http://www.nissanforums.com/sr-series-engines-de-ve/18227-evap-canister-vent-control-valve-closed.html

Intermittent mysteries tend to be related with connector oxidation. Cleanup job...