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Chevy Caprice Failed Smog Test for High NO

My Chevy Caprice was recently smogged and failed the NO test at the 25mph. The measured reading was 804. The catalytic converter was only about 3 years old and a new O2 sensor was put in my Chevy Caprice about 2-1/2 years ago. I took it to a muffler shop and replaced this defective catalytic converter and they informed me that it failed because the vehicle was running rich. After replacing the catalytic converter, I took my vehicle back to get it retest. This time it passed but the NO reading at 25mph measured 534. The max is 542. Can you tell me why did my vehicle barely passed the NO test even with a new catalytic converter? Are there other factors involved?

Answer:

High NO is typically cause do to a "lean" fuel condition as opposed to "rich". Rich fuel conditions will usually lower NOx emissions. NOx (Nitrous Oxides) is created only when an engine's combustion chamber temperatures reach over 2500F. A lean fuel condition will not provide the engine combustion chambers enough fuel to lower cylinder temperatures. We recommend inspecting your vehicle's engine for vacuum leaks, lean fuel conditions (using the "fuel feedback" method) and possibly high engine compression. For more details on high NO faults visit: What Causes High NOx? posted by SmogTips Support

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