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Failed Smog Test

GMC Suburban 5.7L Gas Engine Failing for High NO

1995 GMC Suburban (OBD 1) with a 5.7l gas engine. First the car failed for high NOX so I replaced the EGR. Then it failed for both high hydrocarbons and NOX. My 1995 Suburban with 171,000 mile has always passed smog but has always been near maximum HCs. (Note that the catalyst was replaced at 124,000 miles.) Suddenly the NOX was very high (about 760) and the hydrocarbons were lower than usual (about 70) at both speeds. Since the EGR was replaced 3000 mile ago I checked it. It had a slow leak so I replaced it again with one that held vacuum better. I also ran almost a quart of water through the engine to clean carbon out of the cylinders. The retest (also at a test only station, but a different one) now showed high HCs (about 130) at both speeds and high NOX ( about 710) at both speeds. I have no idea what is going on. It seems like it could be either the O2 sensor or the catalytic converter and I am now just replacing parts arbitrarily. Your best guess would be very much appreciated. Thank you.


You're going to need to look into fuel control... oxygen sensor. We are going to assume your vehicle's CO reads are very low. Though this may appear to be a good thing, in terms of emissions reduction, it may not be. The catalytic converter needs a small amount of fuel for efficient emissions reduction. When a vehicle runs lean, the CAT is not able to perform its functions properly. We recommend inspecting your GMC Suburban's fuel control system and ensure the system is not running lean. Read more about what causes high NOx emissions.

posted by SmogTips Support

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