You should have taken it to the Nissan dealership instead of having your
"mechanic" guess at the problem.

I could have fixed this problem for ZERO dollars in parts, that's right
Nuthin , nada , zip, zilch .

I see this problem quite often and it can be fixed with 2 tools.

an 8mm allen wrench and a wire coat hanger ..... imagine that

My "Check Engine" light is driving me mad. I've been to 4 different shops,
including a Dealer, and nobody has been able to permanently clear it.

I just bought a CarChip E/X at a local auto parts store. Pretty cool
little device.

Anyway, I currently have two trouble codes...

P0402 - Exhaust Gas Recirculation Flow Excessive Detected
P0304 - Cylinder 4 Misfire

These are actually the two codes that initially set off my Check Engine
light back in October. I had a hell of time getting it to pass smog, but
finally got passed two weeks ago... after blowing +$1700.

Here's the detail of the parts that have been replaced, in order:

1) EGR Solenoid
2) EGR Valve
3) Throttle Control Valve
4) A couple of cracked vacuum lines
5) O2 Sensor (upper one... not sure if there's a 2nd one)
6) Catalytic Converter (OBD II cat)
7) Spark Plug wires (I just did these today... they were due anyway.. will
changed the spark plugs tonight/tomorrow)

After getting items 1, 2 & 3 (above), the truck was failing smog with very
high NO readings. My mechanic then changed out item 4, and ran a pre-test
smog check. It passed with flying colors. Unfortunately, that shop
couldn't run the real test because they cannot certify Gross Polluters,
which mine was.

The very next morning, I ran the truck over to someone that can certify
Gross Polluters... And it failed for high NO reading, again. This mechanic
asked me to let him diagnose the problem and he would guarantee that he
would find the problem and get it to pass. He told me that my Catalytic
converter was history. So I let him install a new one. Sure enough, it
passed smog... Finally :-).

3 days later the damned Check Engine light is back, the
idle is very rough, and the truck is really sputtering at speeds under
20mph. Also, I'm getting horrendous gas mileage.

So, anyone have any guesses on what's going on here? I still have
$500-$600 dollars till I throw in the towel and head over to the Toyota
dealer.

This webpage might give you a better ideal as to what was causing your
PO402 diagnostic code...

http://www.obd-codes.com/trouble_codes/generic/p0402-exhaust-gas-recirculation-f
low-egr-excessive-detected.php

Check for a loose or disconnected vacuum line. Start your motor and let
it run. Listen for any hissing sound coming from the engine bay as the
motor is running. BE VERY CAREFUL you don't get any items of clothing or
 hanging jewelry near the fanblade or pulleys as the engine is running.
You could hurt yourself if you do.

If there are no loose or disconnected vacuum lines, then plug your
Carchip E/X device back into your diagnostic port of your truck. Your
motor should kick out some sort of diagnostic code & give you an idea as
to where exactly to check to see if you can fix your problem.

The previously mentioned link above might be able to help.

ID

Just a follow up in case someone else experiences the same.

Took it to a different dealer and they recommended a good cleaning of the  
intake system. I only have a few miles on it right now, but I can report  
that the sputtering/hesitation is completely gone; She's accelerating  
super smooth. I think they may have actually fixed it.

On a somewhat related noted, the dealer I went to had some of the best  
prices and customer service I've experienced. They totally restored my  
faith in Nissan and their dealers :-) I'm stoked.