Diagnosing a Failed Emissions Test – High HC, CO and NOx Causes and Repairs

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In this video I go over how you can diagnose excessive amounts of Hydro Carbons (HC), Carbon Monoxide (CO) and Nitrogen Oxide (Nox) in your exhaust. I go over how a rich or lean condition can cause one or more of these gases to be high and therefore causing you to fail emissions. Also I go over some components like the Catalytic Converter, Oxygen Sensor (O2), Ignition Coils, Spark Plug Wires, Spark Plugs, EGR Valve, which are part of your emissions systems on your car and how they effect different levels of each gas.

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Here are some other video’s you may find interesting:

How to Test an Oxygen Sensor: https://www.youtube.com/watch?v=NKsS6AELPuM&list=PLaRuYYPxjXI2phrxwaDfgykxs3UvigUAg&index=29
How to Test a Catalytic Converter: https://www.youtube.com/watch?v=Tj5NX1Ipfag&list=PLaRuYYPxjXI2phrxwaDfgykxs3UvigUAg&index=30
How to Find a Vacuum Leak: https://www.youtube.com/watch?v=qTzDY95aqXg&index=6&list=PLaRuYYPxjXI2phrxwaDfgykxs3UvigUAg

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Due to factors beyond the control of Ratchetsand Wrenches, it cannot guarantee against unauthorized modifications of this information, or improper use of this information. Ratchetsand Wrenches assumes no liability for any property or personal damage that may arise from doing a repair on your vehicle after watching any of my repair video’s. Due to factors beyond the control of Ratchetsand Wrenches, no information contained in this video shall create any express or implied warranty or guarantee of any particular result. Again Any injury, damage or loss that may…



  • failed my smog test due to my obd ll not ready. no engine light on. I have driven my chevy s-10 over 400 miles since my repair. got any solutions thx.

  • Would you recommend Seafoam?

  • hi. i have a 98 infiniti that failed smog with high NO at 15mph. everything else in smog test passed. there is no check engine light. if you are near san jose i can bring my car to you. can you fix it? clad94538 at gmail thank you.

  • my dads old wagon  oldsmobile custom cruiser is not passing smog on the hc side and I found that the egr does not hold vaccume atleast by pushing in the diaphragm and plugging the vacc outlet ,the diaphragm comes back down it does not stay up.no cel on and wagon runs great.will the egr cause the hc issue?should he buy new plugs ?the smog guy said it can be a cat problem also .any ideas?thanks.

  • great video man… the way you explain poor test results to the problem is phenomenal! thank you

  • this is a pretty good explanation, however it is incomplete. one big thing that most people often over look is carbon build up which can increase cyl pressures and temp's leading to elevated nox emissions, another thing is a rich condition can do the same thing because the cyl pres be higher, along thos same lines is if the head has been machined after a bad warp. on some vehicles i have observed that too much fuel pressure can actually cause high hc's instead of co. every car is different but still a very good video

  • Thank you very much,great video and explanation, my 1998 Bmw 740il failed smog with high nox at 2500rpm, if you are in california I will ring my car to you.

  • hi, iv a 2002 jeep Cherokee 2.5 crd sport, my engine blows dark smoke when I sit idle n rev hard. also uses lot fuel. also when driving feels slow under power as if it's been pulled back as you want to go. don't feel right. has it a turbo on it , or some sensor that may cause it to smoke or under powered.

  • A A

    Very good expanation

  • Hi first of all thank you very much for this video i got so much knowledge by this video i always check videos relating to car fix but never seen someone explaining like this .
    i have one question i drive 1.4 petrol car 3 months back engine warning light came up there was a problem with o2 sensors but i replaced 2 sensors but stll engine light stays on few days later i reset that light but next day engine warning light came back again brought to garage they checked there was a problem with exhaust was leaking they fixed it and last week i brought my car to pass emission test they failed because of high Hc and co and the warning light came back again.so can you please tell me what could be the problem thank you.

  • Why do I have High CO at idle but perfectly fine at revs? Anybody know?

  • I had failed my smog test, and after seeing this video I knew what to do.  My HC had failed, so I Knew it was the air mixture.  I got a new air filter.  I also got the "catclean" product…more cleaning.  I then removed my negative battery terminal to reset computer.  I even got more air in the tires.  My HC was 145 when I failed, today it was at 6.  Easy pass.  Thanks for great video.

  • I have a 1995 Mercury Mistique with a 2.5 V6. The passages in the plenum behind the gasket of the throttle body were completely clogged and the EGR tube had a split in it so I had multiple things wrong and this caused me to fail the emissions test.

  • I like the way you explain, I got it clear.

  • I just subscribed man. Excellent videos!

  • I have 1997 toyota 4runner and it fail smog because of high NOx but it only fail at 15 mph it was high but on 25 mph its low and it pass smog

  • I have a toyota camry1992, it fails the smog, with high HC , should install a new cat on it ? thanks

  • good info. can you tell me how clogged air filter would affect with high hc or co? The maf sensor is going to measure whatever air comes in through the filter so if a filter is clogged then the maf sensor would just measure whatever the amount of air there is and fuel will be delivered according to that.

  • I have been an automotive tech for 47+ years and that has to be one of the best exhaust emission explanations I have ever seen anywhere.  I just subscribed to your channel.

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