Question about 1991 GMC C1500

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Emission test 25/25 hc ppm read 00170 allowed 00143, no ppm read 02342 allowed 00905. 50/15 test hc ppm00195 allowed 00147, no ppm 02959 allowed 00990. This is on a 1994 Z-71 chevy truck 350 motor. Would you say this is a catalytic converter? Help me so I don't go spending a lot of money guessing. Thank you, Frank Perna

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  • fpdouglasvil Apr 29, 2011

    my EGR valve is only about a year old this is on a 1994 chev truck

  • fpdouglasvil Apr 29, 2011

    my EGR valve is only a year old cat converter is 5 years old

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  • There are a couple things here that could be the issue. One would be your Catalytic Converter. The other would be your 02 sensors. It is possible you are running just a little bit rich. There is a sensor on the exhaust not far off the manifold (this is called the upstream sensor). There is another 02 sensor down the exhaust a bit by the Converter (this is called the downstream sensor.

  • Most of the time you will know when the 02 sensor is bad because your fuel mileage will go way down. Often times a check engine light will come on as well (but not always). A failure 02 sensor will run a vehicle rich and that left over fuel from combustion can cause pollution and a failure of an emissions test. However, if you are not loosing any fuel mileage, and you don't have a check engine light on I would be inclined to think that the converter has passed it's point of functionality and needs to be replaced.


Regards, Tony

Posted on Apr 29, 2011

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I would start with the EGR valve. The Cat very rarely goes bad. HIgh Nitrous is a sign the EGR valve is bad or sticking.

Posted on Apr 29, 2011

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