Abstract: An aircraft fire extinguisher system includes a fire extinguisher bottle and an attached explosive squib which has one side connected to ground and its other side connected alternately to two constant current sources in opposite senses. A differential amplifier subtracts the voltages in two sample-and-hold circuits caused by the opposite currents to eliminate the effect of the voltage at the point where the squib is grounded. A utilization circuit, coupled to the output of the differential amplifier, is used to indicate the presence of a fault in the squib or in associated circuitry.