1997 Nissan Altima HIGH NOx

1997 NISSAN ALTIMA
170,450 MILES • 2.4L • 4 CYL • 2WD • AUTOMATIC
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OHYEA
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Recently went to go do state emission test and failed twice, once because of p0171 code with high NOx to which the code is now fixed because I found the air leak leading to a hose connected to EGR vacuum modulator. Thinking I would pass second test I did not. EGR valve (which is working by completing multiple test on it) and its passage way have been cleaned, what else is the reason to a High NOx ?
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Mar 18, 2015 at 7:51 PM
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CATAFINA
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Could be the catalytic converter or a computer control system that's staying in the open loop
Mar 18, 2015 at 9:05 PM
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OHYEA
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I dont think my catalytic convertor since it was replaced 4 years back and would cause a check engine trouble code. What do you mean by computer control system?
Mar 18, 2015 at 9:16 PM
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HMAC300
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your engine is controlled by sensors and a computer taking different readings from all different sources. scan for codes and check fuel pressure with a gauge auto parts rent it.
Mar 19, 2015 at 5:55 AM
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OHYEA
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No codes, can you explain what fuel pressure has to do with high NOx?
Mar 19, 2015 at 10:11 AM
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HMAC300
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i t might be running lean and low fuel pressure can make it do that. however on next testing change oil and filter drive for 20 miles on eway then get it tested. fuel pressure should be 43 psi tested at rail
Mar 19, 2015 at 12:16 PM
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OHYEA
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Ok, I will test fuel pressure, and yea I already changed oil day before testing, then drove 30 min on highway before I went to go take vehicle to get tested
Mar 19, 2015 at 1:06 PM
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OHYEA
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Was checking out parts on rockauto.com and noticed that the EGR vacuum modulator had a type foam or filter in it, but mine does not... Could that lead to high NOx ?
Mar 19, 2015 at 2:57 PM
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HMAC300
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no but if the egr is partially open it would
Mar 19, 2015 at 4:44 PM
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OHYEA
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Will low coolant cause High Nox? Like the numbers shown in the picture of emission results.
Mar 19, 2015 at 9:25 PM
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HMAC300
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If you have low coolant you probably have a blown head gasket have coolant system pressure checked for leaks. I have no further comments or suggestions.
Mar 20, 2015 at 7:07 AM