Smog Check Failure Due to Excessive Carbon Buildup on Cadillac Seville STS

Excessive carbon build up in the engine combustion chambers may be the cause of a smog check failure due to excess NOx emissions. A "top engine" decoarbonization cleaning procedure will commonly correct this concern.

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Average mileage: (–)
2 model years affected: 1991, 1992
2 people reported this problem
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