Failed emissions test with high NO?

Status
Not open for further replies.
Joined
Sep 23, 2008
Messages
12,088
Location
Ontario, Canada
My Tracker just failed its emissions test, with 932ppm of NO, 757ppm allowed.
Hydro Carbons ppm was 3, and CO% was 0.00
The ECU has stored OBDII code P0031, which means low current draw on the pre-cat Oxygen sensor heater element.
It passed two years ago with the same code on the same oxygen sensor, and as its tested on a dyno and I assume the engine is worked hard enough to get the oxygen sensor hot enough to measure correctly.

High NO means I've got a lean mixture right? Do cold O2 sensors read richer or leaner than the actual condition?

I guess my plan is to buy a new O2 sensor and retest, anything else I should look for?
 
Make sure the EGR valve and passages are completrly clear (if equipped) Combustion chamber deposits will also cause high NOX readings
 
If you haven't done a coolant change/flush now may be the time. Localized hotspots could contribute to your issue. But I agree with the o2 sensor as well.
 
All that code means is there is a problem with the heater circuit in that sensor. It will be most prone to causing problems on a cold engine, and while idling.

High NO is usually a problem with EGR/Lean conditions/excessive engine temp/excessive ignition timing.

A platinum O2 voltage generating sensor usually trends to lean when going bad (this makes the engine run richer)so that particular item would not be my first suspect for excessive NO, but it needs to be pinpoint tested and possibly replaced if the heater circuit doesn't ohm out.
 
Last edited:
High NOx usually is caused by a malfunctioning EGR system as other people have mentioned. Is your vehicle equipped with an EGR valve? A faulty reduction catalyst could cause this too. It is possible for the oxidation portion of the cat to still be working efficiently while the reduction portion is not. If you have access to an infrared pyrometer you can check cat temps.
 
Originally Posted By: punisher
All that code means is there is a problem with the heater circuit in that sensor. It will be most prone to causing problems on a cold engine, and while idling.

High NO is usually a problem with EGR/Lean conditions/excessive engine temp/excessive ignition timing.

A platinum O2 voltage generating sensor usually trends to lean when going bad (this makes the engine run richer)so that particular item would not be my first suspect for excessive NO, but it needs to be pinpoint tested and possibly replaced if the heater circuit doesn't ohm out.


In addition to a malfunctioning EGR system it could also mean the Cat Converter is not working up to par.
 
Originally Posted By: demarpaint

In addition to a malfunctioning EGR system it could also mean the Cat Converter is not working up to par.


Yes sir. Working in a dealership, I didn't run across tired cats very often and that tends to bias my diagnostic routine.
 
NO is not mixture related. It is heat related.

CO is generally mixture related, and HC is misfire related.

An EGR system that is not working [even if it is a passive type]causes high NO readings.

But it could also be a weak cat.
 
Thanks for the help everyone, I will check the EGR system as well as replace the O2 sensor, hopefully its not the cat.
The coolant was flushed and replaced at 120,000km, new plugs then too, and the engine works pretty hard with almost no idling so I don't think it should have combustion chamber deposits. I also give it the italian tune up once a week too.
 
Letting all 127 ponies out of the barn for a trip through the gears... I have no idea if it really does anything useful but it can't hurt either for the engine to see the redline once in a while. It must flush a few deposits free from oil and coolant passages, maybe in the exhaust system and cat too?
 
Well, I replaced the O2 sensor with a direct fit NGK, and took the Tracker in for a retest and it passed almost suspiciously well. NO reading went from 932 ppm to 1 ppm? Everything else stayed the same.
This time I did a few hard runs up through the gears before the test, last time I just drove in normally with the water temp not quite up to normal running temp. They say it shouldn't matter but I wasn't taking chances for the second test.
 
I'm glad you solved the problem without spending tons of money and just throwing parts at the problem.

For future emission tests, operating temp does matter, as cat. converters need to be at operating temp to function properly, any competent emission place would know that. So next time do a 30 min. trip before you go in, and make sure that if you have to wait for the test longer than 5-10 mins. you idle the car in the parking lot, especially in winter, to keep cat. converters hot.
 
They say they have to let the car warm up and I guess the exhaust and water temp have to be at good temp before the machine will even take readings... I subscribe to your thinking though, bring it in hot.
The etest place I go to just runs tests, no diagnostics or repairs, so they don't have much incentive to skew the results to failure. They only charge half price on a retest too.
Anyways, I'm safe for another 2 years!
 
Originally Posted By: IndyIan
NO reading went from 932 ppm to 1 ppm?

I recently changed the Ox sensor on my Volvo, and it fixed a high idle-only CO/HC problem. But NOx remained unchanged at 56ppm.
 
Dilution (whatever that is) was something like 9 on the failing test and on the retest it was 15 I think. How that changes I do not know.
 
Status
Not open for further replies.
Back
Top