A method for operating a communication network of a motor vehicle, wherein the communication network comprises a plurality of control devices (10, 12, 14, 16, 18), which are connected to a data bus and can exchange messages with one another via said data bus, wherein the plurality of control devices (10, 12, 14, 16, 18) - is connected to a voltage source; - has a voltage measuring device for determining a voltage (U 10, U 12, U 14, U 16, U 18) applied to their supply connections; and - can access a central and/or internal fault memory at least in a writing manner, in order to document therein information concerning a fault detected by the corresponding control device (10, 12, 14, 16, 18), wherein the method comprises the following steps: a) determining the global supply voltage (U 5) of the voltage source; b) checking whether the determined global supply voltage (U 5) exceeds a predefined upper voltage threshold or falls below a predefined lower voltage threshold; c) providing information representing the determined global supply voltage (U 5) and/or information representing the fact that this exceeds or falls below a threshold to the control devices (10, 12, 14, 16, 18); d) when a fault is detected by a particular control device (10, 12, 14, 16, 18), checking, by use of the provided information, whether the global supply voltage (U 5) exceeds or falls below the voltage threshold in question; e) in the case that the global supply voltage (U 5) exceeds or falls below the voltage threshold in question, the corresponding control device (10, 12, 14, 16, 18), when detecting a fault, instead of information representing the detected fault, writes into its fault memory information representing the fact that the global supply voltage (U 5) has exceeded or fallen below the voltage threshold in question as a possible cause of the fault. - is connected to a voltage source; - has a voltage measuring device for determining a voltage (U 10, U 12, U 14, U 16, U 18) applied to their supply connections; and - can access a central and/or internal fault memory at least in a writing manner, in order to document therein information concerning a fault detected by the corresponding control device (10, 12, 14, 16, 18), a) determining the global supply voltage (U 5) of the voltage source; b) checking whether the determined global supply voltage (U 5) exceeds a predefined upper voltage threshold or falls below a predefined lower voltage threshold; c) providing information representing the determined global supply voltage (U 5) and/or information representing the fact that this exceeds or falls below a threshold to the control devices (10, 12, 14, 16, 18); d) when a fault is detected by a particular control device (10, 12, 14, 16, 18), checking, by use of the provided information, whether the global supply voltage (U 5) exceeds or falls below the voltage threshold in question; e) in the case that the global supply voltage (U 5) exceeds or falls below the voltage threshold in question, the corresponding control device (10, 12, 14, 16, 18), when detecting a fault, instead of information representing the detected fault, writes into its fault memory information representing the fact that the global supply voltage (U 5) has exceeded or fallen below the voltage threshold in question as a possible cause of the fault.