What causes high NO2 levels?

Hey guys, just did an emissions test and the car failed, the reason?, high NO2 levels. Apparently 387 ppm is the max allowable limit for NO2, but my car set a reading of 435 ppm, which is minimally high. The guy said that number usually goes up when the combustion chamber is operating at higher temps. Now I had been driving for about an hour before the test whcih didn't help. I told him that the car passed the last time (two years ago) since then I have changed my cat, spark plugs and wires, O2 sensor, IAC valve and cleaned the throttle body. He said the new plugs may create higher temps as it is burning better, but that I should check the EGR valve for proper operation. So, I am planning to see if the EGR valve is stuck and clean it out abit, plus bring the car in when its cooler. What do you guys think is causing the the high NO2 levels, what about the post O2 sensor?

New plugs wouldn't cause this. Could be the EGR. Also, may want to have the A/F measured with a wideband O2 sensor. If you're running lean, it could cause high combustion temps.

I also wouldn't overlook the new cat converter. Is it stock? Is it a "race cat". Could be it's not working properly.

Stock cat replacement has been installed since last spring, so its not "new new". Aside from replacing it, how can tell if not working properly? Also, not sure waht A/F means?

BMD wrote:

Stock cat replacement has been installed since last spring, so its not "new new". Aside from replacing it, how can tell if not working properly? Also, not sure what A/F means?

Whose cat did you put in? Many "off brand" cats are actually mostly zeolite adsorbers (not much precious metal content - platinum, rhodium or palladium - put "zeolite NOx adsorber" into Google and search to find more) and don't really work all that long to begin with depending on how you drive.

No Idea what cat it was, the shop installed the unit they sell for my application. So you think the cat might be the issue?

Wouldn't hurt to call the shop and ask them if they think the cat could cause the high NO2 levels. Maybe they've seen this before?

When's the last time you passed inspection?

Was the car VERY warmed up when having the test? Around here the minimum run time is 20 minutes, but it would be even better to pull in after a 30-40 minute spirited drive, to make sure the Cat Conv. is nice and hot.

Your state does sniffing on 1996? Weird... Anyway, do you have a scanner?
Unfortunately NOx emissions is something that the PCM has no direct way of reading so other than standard EGR operation, it cannot make any compensations. I agree that you should start looking for things that can cause high combustion temps, starting with the EGR its self (although I would expect to see a code before it causes an emissions failure) and then with a scanner, check fuel trims for being maxed out. Another idea is that if you have some room to potentially produce a few more hydrocarbons, you could maybe go to colder spark plugs too. I'm just brainstorming some ideas here.

Thanks for all the responses guys...

@ Aaron - if you knew the owner of the shop you wouldn't expect him to take any responsibility for a bad cat. Also, the car passed its last test two years ago.

@Derek - The car was driven for about an hour and a half before the test. I also thought that a really hot cat would produce better numbers but the guys suggests doing the re-take when the car is still relatively cool.

@Codith - Up here in this province, cars over twenty years old are exempt, all others get tested every couple of years.
The guy at the testing facility also thought the new plugs could be a reason that the numbers were so high, but also said to look at the EGR valve. Don't have a scanner, but I can get a freeze fram of my fuel trims with my code reader, if they are off how can I bring 'em back to proper levels?

The long term fuel trim (LTFT) would pretty much have to be locked at +16.4 at idle for this to come up because then the air fuel ratio would be leaner than what the PCM can adjust against and giving you a lean (hot) burn. If that is observed in the scan or freeze frame, then you need to look for vacuum leaks, maf performance, O2 sensor performance, stuff like that. But don't worry about that unless you do see fuel trim issues.
As for the colder spark plug thing, it would be more of a trick to counter a hot combustion temp issue. That's not to say that the spark plugs that you have in there are incorrect.
I would definitely check out that EGR though and the plumbing for it.

Do a check of all vacuum lines, a cracked or loose hose can be an issue.

BMD wrote:

No Idea what cat it was, the shop installed the unit they sell for my application. So you think the cat might be the issue?

Aaron's right - ask the installing shop about the cat.

That said, the Walker cats tend to be quite good.

Albertj

Quote :

Aaron - if you knew the owner of the shop you wouldn't expect him to take any responsibility for a bad cat. wink Also, the car passed its last test two years ago.

It's not really about pointing fingers at this point. You want to gain any knowledge you can about the cat. Your mechanic should understand you asking about it.

If the only thing you changed since the last check is the cat, I'd be suspicious that it's the cause of your issue.

The guy that installed the cat isn't my mechanic, they are a local muffler shop with decent prices, obviously I got what I paid for. The cat isn't the only thing I changed since my last pass though. I changed the plugs and wires, the pre O2 sensor, IAC valve, LIM gasket and cleaned the throttle body. Anyway just did the test again and it passed. Of the allowable 387 ppm, my car tested 372 ppm, just made it. What I did was clean the EGR valve which did feel slightly sticky. Also cleaned the passage behind the throttle body that leads to the EGR and changed the PCV valve, and the car was not driven as long this time. Anyway, I obviously still have a problem. Hopefully its the cat thats causing NO2 readings to be that high, which should improve with a quality unit or I am running lean and need to figure out the reason. Thanks for the responses guys, cheers.

BMD wrote:

The guy that installed the cat isn't my mechanic, they are a local muffler shop with decent prices, obviously I got what I paid for. The cat isn't the only thing I changed since my last pass though. I changed the plugs and wires, the pre O2 sensor, IAC valve, LIM gasket and cleaned the throttle body. Anyway just did the test again and it passed. Of the allowable 387 ppm, my car tested 372 ppm, just made it. What I did was clean the EGR valve which did feel slightly sticky. Also cleaned the passage behind the throttle body that leads to the EGR and changed the PCV valve, and the car was not driven as long this time. Anyway, I obviously still have a problem. Hopefully its the cat thats causing NO2 readings to be that high, which should improve with a quality unit or I am running lean and need to figure out the reason. Thanks for the responses guys, cheers.

If you passed you passed... I'd not necessarily quit chasing the cat but I'd focus somewhat more on fuel economy and performance and somewhat less on the cat.

Albertj