The most common fault which would cause your Volvo to produce high NOx is a malfunctioning EGR valve/system. The EGR valve is responsible for re-circulating a small amount of exhaust gas back into the combustion chamber. This is done to cool down the chamber temperature by producing an inert gas along with the regular fuel mixture. Keeping the chamber cool reduces NOx emission. You might want to have a smog check repair station inspect your Volvo's EGR system for proper operation.
Visit this link for proper EGR valve operation: How Does an EGR Valve Work?
Other reasons which could cause your Volvo to produce high NO include: Bad engine cooling, lean fuel condition and high mileage (carbon build-up).
Bad Engine Cooling - If your Volvo's engine 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 Volvo's temperature gauge is always normal and that your cooling system is working properly. If your vehicle has an overheating problem, be sure to perform the necessary repairs.
High Cylinder Compression - Carbon build-up within your Volvo's engine 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.
Lean Fuel Mixtures - Lean fuel mixtures cause high NOx also. A lean fuel mixture is when the engine receives less fuel then is necessary to obtain efficient combustion. Lean fuel mixtures are often due to vacuum leaks. These leaks will allow the suction of additional and un-metered air 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 are often hard to diagnose as they may be present at locations not easily viewed.
