Here's my first 'help' post. My wife's 02 Quest has had the CEL/MIL lit for a very long time. Until the last few months, there were zero driveability issues at all. While it previously could, now no OBDII scanner has been able to communicate with the ECM to pull codes, it does however, power up. The cig lighter fuse and plug both work and there's nothing plugged into the socket when I attempt to read codes. When checking fuses, I find no power to the 7.5A "ENG CONT" fuse found on the right bank of fuses, 5th position from the bottom. A new known-good (continuity confirmed) fuse does not change anything. The other 10A "ENG CONT" fuse has power. The fsm I downloaded from here lacks the fuse box section, so I'm not able to dig any further at this point.

I have previously pulled the dash half apart and switched the ECM for a junkyard sourced one out of a 99 Merc Villager with zero changes noted, still ran perfectly with CEL light still on. The correct ECM was reinstalled. I'd thought the ECM finally crapped out when the van died on my wife one day. I turned my attention to the timing belt, and found the problem. A new belt had us on the road again.
The van now has a idle that seeks, goes so low as to cause a heavy shudder and poor starting, but intermittently and more pronounced when cold.

Before the CEL light problem, I had found that something in the road had been struck while driving, making a nice dent in the oil pan, but more pointedly damaging the rear O2 sensor connector and wiring. I replaced that with a universal style sensor so that I could bypass the broken connector. Although the O2 sensor fuse never blew, I wonder if that could have damaged the ECM directly?

I need to check the wiring between the ECM and that dead fuse. Would one of the big harness connectors underhood be related to this problem?

More poking around has lead me to conclude that the dead fuse noted above is connected to the steering pump pressure that the ECM reads. Gotta dig into the electrical fsm a bit more, now. Still would like to see the fuse box portion of the fsm...

Fuse block diagrams can be found in the foldout section of the FSM. To search for a specific fuse in the EL section, search for No. ##, for example No. 21 gets results for the interior room lamp fuse.

Aha! Didnt think to open that pdf in the list....off to do some more reading! Thanks.

I'll have to back-burner this until the weekend when I get more time. I did find that the 'dead fuse' was No. 12, which indeed is part of the steering pump pressure switch circuit.
In the meantime, what's a known compatible brand of OBDII scanner I could get my hands on? I ask because of reading that some brands don't play well with some makes of cars. Is this an issue with Nissans? One brand I know for sure I tried was an Actron from Autozone. If it's a bluetooth style one, no worries there as I have an Android smartphone I could use.

I have an ELM327 ODBII-to-USB scanner which works fine with my Pathfinder. It's relatively cheap, but the slow shipping can be a real killer. A Bluetooth model is also available and is slightly cheaper.