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Forum > Failed Smog Test > Question

My 1994 Jeep Cherokee 2.5 liter engine failed the smog test for high NOx.

My Jeep passed all parts of smog check but failed the NOx section. The MAX allowed was 527, AVE 170 and my Jeep MEAS 654. Why? And how can it be fixed to pass the test?
Answer:

Here are a few things the consider regarding your Jeep's smog check failure: Your 1994 Jeep Cherokee is not equipped with an EGR valve, and thus the reduction of NOx is the complete responsibility of the catalytic converter. Assuming all other emissions system are working properly, the CAT must reduce NOx emissions. First however we must know what the other emissions numbers (CO and HC) look like. If both CO and HC are within average limits, we can assume the CAT is defective, is not lowering NOx as needed and caused your Jeep to fail the smog check. If either HC Hydrocarbon or CO Carbon Monoxide is borderline failing, there may be emissions faults other then or as well as the CAT. Two other common issue which create high NOx besides the CAT are... Your Jeep has a lean fuel mixture and is producing high NOx. This should be indicated by extremely low CO results. The lean fuel mixture may be due to vacuum leaks or defective fuel control (bad oxygen sensor, or other emissions sensor). And the second common fault; Bad engine cooling. Inefficient engine cooling will cause high NOx as well. Remember NOx (Nitrous Oxides) is created when an engine's combustion chamber temperatures exceed 2500F. posted by SmogTips Support

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