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Nissan Maxima failed smog test for high HC and NOx.

2005 Nissan Maxima with 144,000 miles. I failed my smog check (emissions test fail). I took my care to get a smog check. I passed the visual and function checks, failed the emissions check. Test show High Levels of HC @ 15 and 25 mph. My levels were 75 HC (15mph) and 58 HC (25mph). Need to be under 50 and 34 for HC level I was told. Lastly, my NO level for 15mph was High. It was 693. Must be under 414 NO for 15mph. My CO levels were good and my NO level for 25mph good. What can it be? Was told maybe spark plugs, distributor cap/wires. Please help!! Thank you!
Answer:

Typically when all three emissions are high (HC, CO & NOx) we suspect the Catalytic Converter is defective; but only after the entire emission system is diagnosed. You have to be careful of technicians suggesting... "may it's the plugs, maybe it's the oxygen sensor". You'll end up spending a lot of money on unnecessary repairs. We realize you mention CO is ok, however simply because CO has not failed or appears to be within range does not mean air/fuel ratio is correct. A slightly lean or rich fuel mixture (low CO or high CO respectively) will disrupt fuel combustion, resulting in high HC and even high NO. We can only recommend you have a smog check repair station conduct a thorough diagnosis in order to find out the exact cause of your Nissan Maxima's high HC and NO faults. We can mention this however.... usually high NOx is caused by a defective EGR valve/system. And high HC is caused by a defective ignition system (spark plugs, coils, plug wires if equipped). Alec - SmogTips Support posted by SmogTips Support

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