Your BMW 318is is a non-EGR valve equipped vehicle. NO reduction is primarily controlled by proper air/fuel mixture and the catalytic converter. Once all other emissions components are inspected, and either fixed or found to be functioning properly, we may suspect a damaged CAT (Catalytic Converter)… possibly plugged up. Your BMS 318's CAT may have become plugged up if it has suffered a substrate breakdown. This may have happened due high exhaust temperatures and a rich fuel mixture. After first inspecting the fuel and ignition systems, an intrusive CAT test and Back Pressure/Vacuum test may be performed. These tests will determine whether the CAT is operational.
The important thing to remember is if in fact the CAT is damaged, you have to ensure a pre-existing problem is not the culprit. You do not want to install a new CAT only to damage it after a few months. As mentioned a CAT melt down usually occurs due to high CO (Rich Fuel), old age, and extreme heat. It is recommend to suspect your BMW's CAT is damaged only after a complete emissions systems diagnostics has been completed. Although your BMW did not fail for high CO, CO amounts greater than 0.1 is considered high and a rich fuel condition may exist.
