So far I can confirm the following: cat is good (oem), oxygen sensor is good (oem), timing is good (18) on my 91 Civic Si.

My plugs are showing lean in cylinders 1 & 4, which I assume is contributing to an overall lean condition, causing failure with Nox and HC emissions. I've done several searches on this topic here and want to know the effect of having the oxygen sensor (stock Si computer) further downstream from the stock location. I'm running a D16Y8 header where the oxygen sensor is a few inches before the cat as opposed to the location in my original header (close to the block).

I've seen the term 'sluggish' or 'lazy' oxygen sensor response in my searches but want to know what is the effect on the injector pulse. Can this be the cause of my lean condition?

My next change is to replace the stock header and to monitor the plugs for any improvement. Can anyone comment on the above?

 

Put on the ths stock header. Teperature is a variable that makes the O2 work properly. People have used aftermarket header have complained about how the O2 wasn't getting hot enough (old school DC ceramic header seems to be the worst culprit for this). The one wire O2 doesn't have a heater in it like the 4 wires.
Putting the O2 so far downstream as you have probably isn't getting it hot enough to work properly.

That's my guess.

 

After I failed HC, I put on a MSD 6AL and went back, I passed with flying colors. Happened in MD. And this was with a DOHC Zc motor!

 

Doing that isn't going to bring down the NoX. NoX is affected by the combustion temperature. If you are running lean then your combustion temps go up and you have high NoX.

HC's -

-An ignition misfire. If a spark plug does not spark, the air/fuel mixture in the combustion chamber does not burn and then goes out the tailpipe.
-Incorrect air fuel mixture. If there is not enough fuel in the combustion chamber to ignite, the fuel that is there goes out the tailpipe.
-Low compression. If the compression is too low the air/fuel mixture will not ignite and the unburned fuel will go out the tailpipe.

So let's say he's running lean - that explains the NoX and the HC.

He didn't post numbers so it kind of hard to see how much adjustment the HC's need to come down, but if the lean condition is fixed then the HC's should come down a bit.
-A bad catalytic converter. The converter lives in the exhaust pipe and burns unburned gas as it passes through.

 

Here are my readings (GPM):

HC
limit: 1.0000
my car: 1.3330
result: fail

NOX
limit: 2.2000
my car: 2.3229
result: fail

CO
limit: 20.0000
my car: 6.6169
result: pass


I should add that my motor is a D16Z6, stock a6 ecu, rpm activated vtec, z6 fpr, a6 distributor, a6 intake manifold.

Back to my oxygen sensor question: if the oxygen sensor isn't getting hot enough, isn't the ecu not getting enough voltage reading, thus wanting to increase the injector pulse width? Correct me if I'm wrong please.

 

Just a guessing because I have only had lazy and bad O2's. You have a supposedly good O2 that's in the wrong place. Because of it's location maybe it's acting as though it's lazy. Your results look similar to one of mine with a lazy O2... high HC's and NoX alony with above average CO.

You are right about how the O2 should be thinking it's a lean condition when it's too cool. I had to go read up on what it would do if it were cold. They send less voltage when cold or lean and more voltage when working properly and rich.

 

Try setting your ignition timing to 15-16 degrees. Advanced timing contributes to high NoX levels. It may be just what you need since you are so close to the passing numbers. I dont know how this will affect your HC levels. I think its kind of odd that you have high NoX and HC. Also make sure u warm the car up really good before you take it to get tested....and if they make u wait in the parking lot, leave the car idling so the cat will stay somewhat hot.

 

Thanks again for the advice everyone. On my 4th try, I PASSED

What I recently did:
-new oil
-new plugs
-installed stock A6 header in place of Y8 header
-retarded timing all the way (ghetto style, basically loosened distributor, pulled toward front as much as possible, retightened screws)

My buddy reminded me that the stock timing on the D16A6 is 16 BTDC. Mind you my setup is a D16Z6 with stock A6 ecu. Having it at 18 BTDC the previous 3 failures was the cause of the high NOx (and my plugs showing lean). This is probably another reason to have the correct ecu per engine, else go VAFC.

My conclusion:
HC too high? --> replace oil & filter
CO too high? --> replace cat
NOx too high? --> retard timing

YMMV!