Method for monitoring the efficiency of a system for after-treatment of exhaust gas

The method involves injecting a reducing agent into an exhaust line (2). A post-treatment system (5) is provided with a reduction catalyst (6). Performance of the post-treatment system is verified. Conditions of state of components of the post-treatment system are checked. An operation condition of a nitrogen oxide sensor (11) is verified. An operation condition of a reducing agent injector (7) is checked. Temperature of ambient air is determined, which lies between minimal temperature and maximum temperature.