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Forum > Failed Smog Test > Question

2000 Cadillac Catera failed smog check for high NOx.

My Cadillac Catera failed the smog test. It passed all emissions except the NOx category; Test @ 15MPH Max 400 average 57, car registered 619. What would cause this? Plugs, wires, are the injectors dirty. All other sensors etc. passed.
Answer:

It's important to also look at the CO emissions your Cadillac produced. If CO was low then you might have a high NOx problem due to a lean fuel condition. If CO is right on mark or above normal, your Cadillac's high NOx fault may be due to the EGR valve. NOx is caused due to increased engine combustion temperatures. Any engine problem which could cause the combustion chamber temperatures to reach over 2500f will cause high NOx, i.e.. Lean fuel/air mixtures, vacuum leaks, dirty injectors, bad fuel pump or dirty fuel filter, defective EGR valve. You see there are many things which could cause high NOx on your Cadillac. We have to recommend a hands-on diagnosis by a smog check repair station. There are too many emissions components which could cause this fault to simply start replacing things, specially if the fault is as simple as a vacuum leak which may not need replacement of any parts, just a 6"-12" vacuum line... It's possible. posted by SmogTips Support

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