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Ask Mike V. Your Own Question
Mike V.
Mike V., Subaru Technician
Category: Subaru
Satisfied Customers: 25677
Experience:  Certified Subaru Technician, factory trained by Subaru.
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Failed emmisions with high nox. level did not change after

Customer Question

failed emmisions with high nox . level did not change after new cat installed
JA: What is the model/year of your Subaru?
Customer: 2002 outback 2.5 engine
JA: How many miles are on the car? Is it an automatic or a manual transmission?
Customer: 179,000 automatic
JA: Are you fixing your Outback yourself? What have you tried so far?
Customer: new cat new rear o2 ran bg 44k and a tank of 93 octaine
JA: Anything else you want the mechanic to know before I connect you?
Customer: no
Submitted: 2 months ago.
Category: Subaru
Expert:  Chad Farhat replied 2 months ago.

Hi, I'm Chad. Welcome to JustAnswer. I'm reviewing your question and I will be posting your reply ASAP.

Expert:  Chad Farhat replied 2 months ago.

hey how are you

What are the fault codes retrieved ?

Please let me know

Thank You

Chad Farhat

ASE Certified Technician

Customer: replied 2 months ago.
no codes present hc was .0824 limit is 1.2 co was .7380 limit is 15.00 nox was 5.2583 limit is 1.5
Expert:  Mike V. replied 2 months ago.

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.

Please let me know if there is anything else I can do to help.
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Thank you so much!

Customer: replied 2 months ago.
car does not have an egr valve and no vacuum leaks
Expert:  Mike V. replied 2 months ago.

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.

Customer: replied 2 months ago.
all that was checked by subaru they said replace cat and rear o2. i quess i will take back to them
Expert:  Mike V. replied 2 months ago.

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.

Please let me know if there is anything else I can do to help.
Just Answer does not compensate me for my time and resources, so please rate my answer by using the stars at the top of the page. Payment for your question includes any follow-up questions you may have. Free trials and subscription questions still require ratings.
Thank you so much!