A fluid detection system includes a central monitor, an overfill probe for detecting the approach of the overfill condition of a tank of liquid, and n other probes, where n is an integer greater than or equal to 1. The probes are connected to the monitor via electronic cables. The monitor communicates with the probes by producing probe identifier signals comprising integers starting with zero up to the number n. Each probe includes a signal receiving and sending means which compares the identifier signal sent by the monitor to a preset identifier number for that probe and sends a status signal indicative of the fluid status at that probe when it receives the identifier signal that is the same as the preset number. The overfill probe is set with the identifier zero. The monitor includes a timing circuit which determines a six second probe check cycle. The monitor checks one probe every probe check cycle according to the following sequence: probe zero, probe number one, probe zero, probe number two, probe zero, probe number three, etc. until all n probes are checked. The monitor also includes an interrupt mode which may interrupt the probe check function. Whenever the monitor returns from interrupt, it first checks the zeroth probe then continues in the probe check cycle from which it was interrupted.