The disclosure relates to a method for checking and ensuring the functionality of an exhaust gas aftertreatment system of an internal combustion engine. The exhaust gas aftertreatment system has a catalytic converter and a voltage source. The catalytic converter has a catalytic converter area and an electrical heating device which is selectively supplied with electrical energy from the voltage source. The method includes determining an actual value which is characteristic of an ohmic resistance of the heating device. The actual value being determined by using an electrical current strength supplied to the heating device and an electrical voltage supplied to the heating device from the voltage source. The method also includes providing a setpoint value which is characteristic of an expected ohmic resistance of the heating device. The setpoint value takes into account a specific heating-up behavior of the heating device and an expected long-term behavior of the catalytic converter.