Our 1995 Ford E250 van failed smog.

There are several reasons vehicles encounter NOx failures. The most common is a malfunctioning exhaust gas recirculation (EGR) system. The EGR system recirculates burned up exhaust gases back into the combustion chambers. This keeps the combustion chamber temperatures down and reduces NOx emissions. Another high NOx causing problem include: Lean fuel mixtures: A “lean fuel mixture” is when the engine receives less fuel then is necessary to obtain efficient air/fuel combustion and can be caused due to engine fuel management problems and/or vacuum leaks. Vacuum leaks will allow the suction of additional and un-metered air (oxygen) into the combustion mixture or exhaust (depending on where the vacuum leak is located) disturbing pre/post fuel combustion and increasing NOx emissions. Vacuum leaks can be difficult to locate if they are present at locations not easily seen. Fuel management faults will require a diagnosis of your FORD E350's engine control system. Bad engine cooling: Engine cooling problems may cause high NOx also. If your engine's cooling system is not working efficiently, chances are there is an excessive amount of NOx being created. Remember NOx (Nitrous Oxides) is created only when an engine's combustion chamber temperatures reach over 2500F. A bad cooling system will create NOx. For this reason you want to make sure your vehicle's temperature gauge is always normal and that your cooling system is working properly. High cylinder compression: Excessive carbon build-up within your engine's combustion chambers will cause increased engine temperature and high NOx. Carbon build-up normally develops in an engine's combustion chambers over some period of time. High increase in carbon build-up causes increased cylinder compression, which causes high temperatures, which result in high NOx emissions. Keep in mind this problem is usually seen in vehicles with over 200,000 miles. The solution to this problem is called an Engine DeCarbonizing. It usually costs around two labor hours at a smog repair station. It will remove a good amount of carbon of your engine's piston heads and valves. This will increase combustion space, lower compression and lower NOx.