It appears your Lexus LS 400 is a non-EGR valve/system equipped vehicle; which means NO emissions are primarily controlled by proper air/fuel mixture and your Lexus's Catalytic Converter. Once all other emissions components are inspected, and either fixed or found to be functioning properly, we may suspect a damaged CAT (Catalytic Converter)… possibly plugged up.
NOx is created when engine combustion chamber temperatures rise above 2500f. In order for your Lexus LS 400 to pass the smog check, fuel delivery and air/fuel mixture must be preciously controlled to produce the least emissions. If the injectors do not present enough fuel to the combustion chambers, this would cause low CO emissions and high NOx emissions. A smog check repair station will first have to find out if your Lexus is possibly running lean (low CO); meaning it is not providing enough fuel to cool the combustion chamber temperatures down.
Once it is confirmed air/fuel ratio is 14.7:1, the smog repair technician can move on to diagnosing mechanical faults such as the CAT.
How much these repairs will cost will depend on what components will need to be replaced/ repaired. Smog check diagnosis and repairs usually cost $50-70 dollars per hour. California average smog check repair cost is $450.00. Each vehicle's repairs are different and your specific vehicle's smog related faults may cost more or less than the State's average.
