If this Dodge is equipped with an EGR valve, you want to check that system first. Insure the EGR valve opens with vacuum and that its passages are free from carbon buildup. Once you make sure your Dodge truck's EGR system is working, you may move on to diagnosing the fuel feedback system in order to ensure a "lean" fuel condition is not causing the high NOx problem.
NOx (Nitrous Oxides) are created when an engine's combustion chamber temperatures rises above 2500 Fahrenheit. In order for a vehicle's engine to pass a smog inspection, 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.
On a computer controlled engine, such as in this case, there are several emission sensors/components which play a role in determining fuel ratio. Along with the Oxygen Sensor, several components such as the, TPS (Throttle Position Sensor), ECT (Engine Coolant Temperature Sensor) and the MAP (Manifold Absolute Pressure Sensor) sensors also send vital engine information to the computer which is used to determine proper air to fuel ratio. If any one of these components are defective they can alter the fuel delivery program. A smog service station would be able to diagnose each component and report it's condition.
Lean fuel mixture will cause high NOx: Lean fuel, due to vacuum leaks or defective fuel control, will cause high NOx. A “lean fuel mixture” is when the engine receives less fuel then is necessary to cause efficient combustion. Vacuum leaks are open passages, normally due to defective gaskets or vacuum lines, between two engine components.
High compression can cause high NOx: High amount of carbon build-up within your engine's combustion chambers will cause increased engine temperature and high NOx. The solution to this problem is called an Engine DeCarbonizing. It will remove a good amount of carbon off of your engine's piston heads and valves. This will increase combustion space, lower compression and lower NOx.
