Smog Check Failure Due to Excessive Carbon Buildup on Cadillac DeVille

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: 135,500 (113,000–158,000)
5 model years affected: 1991, 1992, 1993, 1994, more1995
7 people reported this problem
2 people shared problem details
1994 Cadillac DeVille158,000
Caddy failed due to NOx being too high on both high and low speed tests
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