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What are the fault codes retrieved ?
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ASE Certified Technician
HI nox is usually the result of a lean condition. This can be caused by vacuum leaks or an issue with the front O2 sensor. That being said, I have also seen egr valves stick open a bit and cause the lean mix. It is possible to have some kind of timing issue or valve issue ( carbon on the valves) cause this problem as well. Most of the time however it's either an egr problem or front O2 sensor problem.
I have seen a lot of front O2 sensors fail without storing a code.
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I guess that leaves the front O2 sensor, fuel pressure and last but not least the timing or carbon on the valve issue. Lean fuel mixture is what is going to produce high nox.
The rear O2 sensor won't cause the lean fuel mixture problem. It's the front one that does the majority for the fuel trim. When you have high nox you basically have an issue with the combustion chambers running hotter then they should. This is caused by a lean fuel mix of gas and air. That front O2 has the most to do with that.