Method for diagnosing a system behaviour of a system component of a cleaning system (16, 200) of a motor vehicle, wherein an output quantity depends on an input quantity by means of the system behaviour of the system component of the cleaning system (16, 200), wherein exceeding an upper threshold quantity by an actual output quantity and/or falling below a lower threshold quantity by the actual output quantity indicates that an actual system behaviour deviates from an expected system behaviour, exhibiting the following steps: - Determine the input quantity; - Determine the actual output quantity; - Retrieve the upper threshold quantity and/or the lower threshold quantity depending on the input quantity; - Compare the actual output quantity with the upper threshold quantity and/or the lower threshold quantity; - Preferably calculate a deviation between the actual output quantity and the upper threshold quantity, if the actual output quantity has exceeded the upper threshold quantity, and/or calculate the deviation between the actual output quantity and the lower threshold quantity, if the actual output quantity has fallen below the lower threshold quantity; and - Store a diagnostic signal if the actual output quantity exceeds the upper threshold quantity and/or if the actual output quantity falls below the lower threshold quantity. wherein an output quantity depends on an input quantity by means of the system behaviour of the system component of the cleaning system (16, 200), wherein exceeding an upper threshold quantity by an actual output quantity and/or falling below a lower threshold quantity by the actual output quantity indicates that an actual system behaviour deviates from an expected system behaviour, exhibiting the following steps: - Determine the input quantity; - Determine the actual output quantity; - Retrieve the upper threshold quantity and/or the lower threshold quantity depending on the input quantity; - Compare the actual output quantity with the upper threshold quantity and/or the lower threshold quantity; - Preferably calculate a deviation between the actual output quantity and the upper threshold quantity, if the actual output quantity has exceeded the upper threshold quantity, and/or calculate the deviation between the actual output quantity and the lower threshold quantity, if the actual output quantity has fallen below the lower threshold quantity; and - Store a diagnostic signal if the actual output quantity exceeds the upper threshold quantity and/or if the actual output quantity falls below the lower threshold quantity. - Determine the input quantity; - Determine the actual output quantity; - Retrieve the upper threshold quantity and/or the lower threshold quantity depending on the input quantity; - Compare the actual output quantity with the upper threshold quantity and/or the lower threshold quantity; - Preferably calculate a deviation between the actual output quantity and the upper threshold quantity, if the actual output quantity has exceeded the upper threshold quantity, and/or calculate the deviation between the actual output quantity and the lower threshold quantity, if the actual output quantity has fallen below the lower threshold quantity; and - Store a diagnostic signal if the actual output quantity exceeds the upper threshold quantity and/or if the actual output quantity falls below the lower threshold quantity.