just took my car in to get smogged, it passed everything but the nox. these are the following readings:

15mph max: 419 average: 57 measured: 713 FAIL
25mph max: 706 average: 50 measured: 614 pass

its a 96 bonneville se (non turbo) with 187k miles. im stupid and forgot to change the oil before taking it in, its gonna be at the 3k mark in about 100 miles. is there an EGR valve on the 96? i read somewhere that the 92 and up dont have egr, they have a 3 part cat instead, and one part of it gets rid of nox. well anyways, what do i need to do to get this thing to pass? the check engine light never comes on, so i was thinking maybe its an egr passage that could be clogged... but i dont know what the egr looks like or where its located. thanks a lot guys!

oh one thing i forgot to mention.. every time i start the car i have to give it a little gas to get it to start. its only been doing this for a few weeks, but once i start it the engine revs up to like 2 or 3 k and then goes back to around 900 rpm. besides this start-up issue, the car runs beautifully.

I got more questions than answers. When was the last time you replaced the spark plugs, ignition wires, air filter, gas filter, pcv valve, and (most of all) your O2 sensor???

I'm not saying that your EGR may be bad, but I figure your MIL(aka SES) light would have come on for that. Also I doubt not changing your oil with 3K on it caused you to fail the emissions test. Cleaning the Throttle Body also couldn't hurt.

spark plugs , wires, pcv are all new.. only replaced a few months ago. the air filter hasnt been touched in a while, its not too dirty.. havent touched the gas filter or the o2 sensor. is there a way i can test the o2 or the pcv to see if they are faulty?

When my front oxygen sensor went bad in my SSEi the car was running dirty rich.
I didn't have a SES light either. I changed the o2 sensor on a hunch and my mpg'* went up about 60-70%.

it says on the smog check vehicle inspection report that the oxygen sensor passed the functional check. i would think that if the o2 or fuel filter were bad that my emissions would have been higher for the hc and co readings as well. but the hc was below average at both 15 and 25 mph speed tests, and the co readings were:

15mph max: 0.48 ave: 0.02 meas: 0.05
25mph max: 0.46 ave: 0.03 meas: 0.03

i assume that the egr is the problem as its only function is to reduce nox, which it doesnt seem to be doing.

Theoretically, if you pull the PCV Valve out and shake it and it rattles, it'* good. But my theory is, if you spent the time to take it out(and you don't know how long it'* been in there) just replace it. One of the benefits of replacing an old 02 sensor, is better gas mileage(along with better emissions). Please note that most members here strongly recommend the Delco brand O2 sensor(with Bosch being a no-no). Good Luck.

could you tell me what the gm part number is that i would need from gmpartsdirect.com?

The GM part # is in techinfo. You'll need the POSN1 o2 sensor. But here it is anyway with the gmpartsdirect prices -courtesy of willwren:


1996 Vin# K,1 - POSN1
GM# 25312184 (AC Delco #AFS-75)
$34.07

Behind Cat POSN3
GM# 25312196 (AC Delco #AFS-97)
$56.94

When replacing the sensors should you replace both or just the front? While we are on the topic of emissions how long do CAT'* last?