Your Porsche 944 is a non-EGR equipped vehicle; which means NOx reduction is primarily controlled by the catalytic converter and proper fuel control. Once all other emission (smog) components are inspected, and either fixed or found to be functioning properly, we may suspect a damaged CAT (Catalytic Converter)… possibly plugged up.
Your Porsche's CAT may have become plugged up if the substrate within it has broken down over time due to high exhaust temperatures and rich fuel. After first inspecting the fuel and ignition systems, an intrusive CAT test and Back Pressure/Vacuum test should be performed to determine the condition of the CAT.
A smog check repair station should first ensure your Porsche 944 is in proper fuel control. The diagnosis should begin with inspecting the fuel injection “feedback” system. This includes ensuring the oxygen sensor (along with other emission sensors, such as the Air Flow Sensor) are working properly, the computer system is receiving the signals from the oxygen sensor, the computer (ECU) is computing the data properly and sending the correct signals to the fuel injectors to either increase or decrease fuel delivery to the combustion chambers. Any fault with the fuel feedback system could be signaling your Porsche 944's ECU to command the injectors to run lean. A lean running engine, whether due to vacuum leaks or defective fuel control will create high NO.
