Del City GFI Receptable Tester and Tru-SparkT Ignition Firing Indicator Del City's GFI Receptacle Tester Detects common wiring problems in standard and ground fault circuit interrupter outlets. The testers plug in and users are able to view the light pattern on the tester for wiring conditions. Pressing the black button tests the operation of ground fault receptacles. Del City also has a Tru-SparkT Ignition Firing Indicator that quickly and accurately tests DIS and other ignition systems. It can also test without piercing or removing spark plug wires.