A fluid overfill detection and control system for controlling the transfer of fluid into a transport vehicle having multiple fluid compartments, the system comprising: a plurality of overfill sensors, each sensor comprising a pulse input, a pulse output, and a diagnostic output, wherein each sensor is associated with a respective vehicle compartment and produces an output pulse on the pulse output in response to a respective input pulse received at the pulse input as a function of the fluid level in its respective compartment, wherein the overfill sensors are arranged in series with one another, including a first sensor and a last sensor, such that the pulse output of each overfill sensor except the last sensor is coupled to the pulse input of the next overfill sensor in the series, characterized in that the system comprises: a diagnostic line connected to each diagnostic output of the overfill sensors; each overfill sensor is configured to connect the diagnostic output to ground in response to receiving a pulse at the pulse input if the sensor is not in contact with a liquid; a sensor count signal source configured to provide a sensor count signal being indicative of a total number of overfill sensors installed in the vehicle; and a controller configured to: regulate the transfer of fluid into the transport vehicle, provide an input pulse to the first overfill sensor, receive an output pulse from the last overfill sensor, receive a diagnostic signal from the diagnostic line, the diagnostic signal being indicative of a number of overfill sensors that generate an output pulse, receive and compare the sensor count signal to the diagnostic signal detected on the diagnostic line, and inhibit the transfer of fluid into the vehicle if the output pulse is not received or if the number of overfill sensors indicated by the diagnostic signal does not match the number of overfill sensors indicated by the sensor count signal. a plurality of overfill sensors, each sensor comprising a pulse input, a pulse output, and a diagnostic output, wherein each sensor is associated with a respective vehicle compartment and produces an output pulse on the pulse output in response to a respective input pulse received at the pulse input as a function of the fluid level in its respective compartment, wherein the overfill sensors are arranged in series with one another, including a first sensor and a last sensor, such that the pulse output of each overfill sensor except the last sensor is coupled to the pulse input of the next overfill sensor in the series, characterized in that the system comprises: a diagnostic line connected to each diagnostic output of the overfill sensors; each overfill sensor is configured to connect the diagnostic output to ground in response to receiving a pulse at the pulse input if the sensor is not in contact with a liquid; a sensor count signal source configured to provide a sensor count signal being indicative of a total number of overfill sensors installed in the vehicle; and a controller configured to: regulate the transfer of fluid into the transport vehicle, provide an input pulse to the first overfill sensor, receive an output pulse from the last overfill sensor, receive a diagnostic signal from the diagnostic line, the diagnostic signal being indicative of a number of overfill sensors that generate an output pulse, receive and compare the sensor count signal to the diagnostic signal detected on the diagnostic line, and inhibit the transfer of fluid into the vehicle if the output pulse is not received or if the number of overfill sensors indicated by the diagnostic signal does not match the number of overfill sensors indicated by the sensor count signal. a diagnostic line connected to each diagnostic output of the overfill sensors; each overfill sensor is configured to connect the diagnostic output to ground in response to receiving a pulse at the pulse input if the sensor is not in contact with a liquid; a sensor count signal source configured to provide a sensor count signal being indicative of a total number of overfill sensors installed in the vehicle; and a controller configured to: regulate the transfer of fluid into the transport vehicle, provide an input pulse to the first overfill sensor, receive an output pulse from the last overfill sensor, receive a diagnostic signal from the diagnostic line, the diagnostic signal being indicative of a number of overfill sensors that generate an output pulse, receive and compare the sensor count signal to the diagnostic signal detected on the diagnostic line, and inhibit the transfer of fluid into the vehicle if the output pulse is not received or if the number of overfill sensors indicated by the diagnostic signal does not match the number of overfill sensors indicated by the sensor count signal. regulate the transfer of fluid into the transport vehicle, provide an input pulse to the first overfill sensor, receive an output pulse from the last overfill sensor, receive a diagnostic signal from the diagnostic line, the diagnostic signal being indicative of a number of overfill sensors that generate an output pulse, receive and compare the sensor count signal to the diagnostic signal detected on the diagnostic line, and inhibit the transfer of fluid into the vehicle if the output pulse is not received or if the number of overfill sensors indicated by the diagnostic signal does not match the number of overfill sensors indicated by the sensor count signal.