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.
Car Problem Reports
Cadillac DeVille Smog Check Failure Due to Excessive Carbon Buildup
Cadillac DeVille Problem
Average Mileage: 135,500 mi (113,000 mi - 158,000 mi)
Visitor, 1994 Cadillac DeVille, 158,000 mi
Caddy failed due to NOx being too high on both high and low speed tests
Car failed for high nox