Nissan Repair Questions? Ask a Mechanic for Answers ASAP
I'd love to help.
Do they tell you what it is failing for on the test?
Carbon monoxide, CO?
Oxides of Nitrogen, NoX?
In my state we test for these three gasses and a failure in one or the other helps indicate where the problem is.
Ok. Well how would you like to proceed? I can give you some pointers about what causes high numbers in each category if you would like. Do you plan to have it tested soon?
It's hard to say without knowing why it failing, but here are some basic pointers:
Hydrocarbons are un-burned fuel. If you have replaced plugs, wires, cap rotors, etc, that will usually solve the hydrocarbon issue. Leaky fuel injectors can also cause high hydrocarbons on this model and the fuel injectors and fuel pressure regulator can be replaced to fix it.
Carbon monoxide is caused by poor fuel mixture and poor catalytic converter function. Again- it seems like you have already changed the cat and o2 sensors.
NoX is caused by a mixture of both hydrocarbons and carbon monoxide changing into oxides of nitrogen. The best way to reduce Nox is the ensure a proper fuel mixture, and to check the EGR system for restrictions. A restricted EGR will cause excess NoX production and foul the catalytic converters. You should also check for a faulty EGR back pressure transducer BPT.
Which of these systems do you want to check into first? I can send some diagrams if you plan to tackle it yourself.
I can't say for certain because I don't know what they test for in your area, but here in my state, if there is a hole in the exhaust system it can still be tested, but might fail for poor sample conditions because of the hole. Basically the emissions machine detects that there is air in the exhaust system diluting the test sample from the exhaust leak.
What sort of pipe/muffler issues do you have?
I just wanted to check in and see if you needed any additional information. I hope that I have been of some assistance and would appreciate a positive rating if it have. Thanks!