Method for differentiating an incorrectly expected concentration from an incorrectly detected concentration of an exhaust gas component of an internal combustion engine, wherein an actual concentration is set by predefining air-quantity-actuating values (LMSW) and fuel-quantity-actuating values (KMSW), comprising the steps: operating the internal combustion engine at various operating points (P1, P2, P3, P4) with air-quantity-actuating values and fuel-quantity-actuating values which are predefined on an operating-point-specific basis, detecting and calculating values of the concentration at the operating points (P1, P2, P3, P4) and/or at the transition between the operating points (P1, P2, P3, P4), determining deviations between the values of the concentration which are respectively detected and expected at an operating point (P1, P2, P3, P4), comparing the deviations with predefined threshold values, generating a pattern of fault bits by setting an operator-point-specific fault bit if a deviation at a specific operating point (P1, P2, P3, P4) exceeds a threshold value and differentiating an incorrectly expected concentration from an incorrectly detected concentration of the exhaust gas component by assigning at least one first pattern to an incorrectly detected concentration and at least one second pattern to an incorrectly expected concentration, characterized by setting fixed air-quantity-actuating values and a first fuel-quantity-actuating value at a first operating point, detecting and calculating first values of the concentration at the operating point, operating the internal combustion engine at the fixed air-quantity-actuating values with a changed fuel-quantity-actuating value (KMSW2) at which the rotational speed of the internal combustion engine rises, detecting and calculating second values of the concentration at a second operating point, defined by a predefined first distance (S1_KWW) at the start of the operation with the changed fuel-quantity-actuating value (KMSW2), at a predefined second distance (S2_KWW) at the start of the operation with the changed fuel-quantity-actuating value (KMSW2), detecting and calculating third values of the concentration at a third operating point, switching off the fuel supply, at a fourth operating point, at which at the location of the detection of the concentration, with the fuel supply switched off, a concentration of the exhaust gas component has been set which corresponds to the concentration of the exhaust gas component in the ambient air, detection and calculation of fourth values of the concentration. operating the internal combustion engine at various operating points (P1, P2, P3, P4) with air-quantity-actuating values and fuel-quantity-actuating values which are predefined on an operating-point-specific basis, detecting and calculating values of the concentration at the operating points (P1, P2, P3, P4) and/or at the transition between the operating points (P1, P2, P3, P4), determining deviations between the values of the concentration which are respectively detected and expected at an operating point (P1, P2, P3, P4), comparing the deviations with predefined threshold values, generating a pattern of fault bits by setting an operator-point-specific fault bit if a deviation at a specific operating point (P1, P2, P3, P4) exceeds a threshold value and differentiating an incorrectly expected concentration from an incorrectly detected concentration of the exhaust gas component by assigning at least one first pattern to an incorrectly detected concentration and at least one second pattern to an incorrectly expected concentration, characterized by setting fixed air-quantity-actuating values and a first fuel-quantity-actuating value at a first operating point, detecting and calculating first values of the concentration at the operating point, operating the internal combustion engine at the fixed air-quantity-actuating values with a changed fuel-quantity-actuating value (KMSW2) at which the rotational speed of the internal combustion engine rises, detecting and calculating second values of the concentration at a second operating point, defined by a predefined first distance (S1_KWW) at the start of the operation with the changed fuel-quantity-actuating value (KMSW2), at a predefined second distance (S2_KWW) at the start of the operation with the changed fuel-quantity-actuating value (KMSW2), detecting and calculating third values of the concentration at a third operating point, switching off the fuel supply, at a fourth operating point, at which at the location of the detection of the concentration, with the fuel supply switched off, a concentration of the exhaust gas component has been set which corresponds to the concentration of the exhaust gas component in the ambient air, detection and calculation of fourth values of the concentration.