Q: Emissions Failure due to Oxides of Nitrogen on 1995 Ford Explorer

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My vehicle just failed emissions testing due to oxides of nitrogen being slightly higher than permissible levels. All other emission test results were well within limits. Can this problem be fixed with just a tune-up?
(1) Answer
NOx is formed when combustion temperatures are too high. Typically a tune up will not correct this problem. Incorrect ignition timing can cause higher than normal NOx. If the car has an EGR valve there could be an issue with that (EGR reduces combustion temps). There could also be an issue with excessive carbon build up creating a higher compression ratio, thus creating higher combustion temps. The other possibility is that the three way catalyst is not working as well as it should. A qualified technician can look at these systems for you and determine what the cause is.