The P2209 trouble code indicates a problem with the nitrogen oxide (NOx) sensor in the catalytic converter. This sensor measures nitric oxide and nitrogen oxide in the exhaust. When the NOx sensor reaches its storage capacity, it triggers regeneration of the fuel management system. The code is set when the sensor's voltage reading is abnormal. Potential causes include a faulty NOx sensor, wiring issues, or a poor electrical connection. Recommended repairs include fixing exhaust leaks, replacing fuel system components, the catalytic converter, oxygen sensor, or sensors related to air and coolant monitoring. It is best to service the problem soon to prevent further damage.