Method for ascertaining the nitrogen oxide fraction and/or ammonia fraction in the exhaust gas of an internal combustion engine

An exhaust tract for an internal combustion engine, the exhaust tract comprising:

• a catalytic converter for aftertreatment of the exhaust gas of the internal combustion engine;
• an exhaust gas sensor arranged downstream of the catalytic converter and configured for generating a signal indicating a content of the exhaust gas;
• and
• a control unit which receives the signals of the exhaust gas sensor and is programmed to execute a method comprising:
• determining an operating state of the internal combustion engine;
• generating a signal using the exhaust gas sensor;
• if the operating state indicates lean operation, using the signal to determine a nitrogen oxide content of the exhaust gas;
• if the operating states indicates rich operation of the internal combustion engine, using the signal to determine an ammonia content of the exhaust gas;
• and
• determining the carbon monoxide content in the exhaust gas at least partially based on the determined ammonia content and a carbon monoxide correlation factor.