Circuit testers I use for whatever, made out of fuse holders, 12 gauge wire. You can plug in fuses, circuit breakers, and test what you like.

One way I use them, is for looking at injectors through the fuse block, this is taken from a 91 Pontiac Firebird 3.1 V6, non sequential injection, with a solid misfire, ignition and other possibilities check out. Looking at banked injectors through the power supply circuits at the fuse block can give you an idea if any are shorted, or open, or slightly out of range.

Bank one set of injectors, this is the total draw of all three injectors combined, each spike consists of all three firing at the same time, not individually, compressing the time base further would allow time for intermittents to show up. Total draw, just over three amps. Injector resistance spec for these are 12.2 Ohms each, so if we apply ohms law, to get amps, 14.2v/12.2ohms=1.17 amps, about 1 amp per injector (injector resistance would be a bit higher with engine at operating temp). This bank seems ok.

Bank two set of injectors, total draw, about 2.2 amps, one of these injectors has high resistance, or may even be open, now you know what bank is causing the misfire, time to pull the plenum.

If any of the injectors were shorted, total draw would be higher on either bank. (if it will run, Ha!)

This can be done on most applications, but there will be problems with some, above is a 97 Chev PU 5.7 V8 Vortec, sequential injection. Other load components on the circuit will show up in the waveform. In this trace, the coil driver and coil draw pattern show up since they are fused on the same circuit. You can still see the injectors firing in the waveform, and they are pulling just under an amp each, you would still see any problem with them in the waveform. They also ride the draw of the other components in the trace since they are added each time they fire. If you trigger the trace with number one firing, you could know which injector has a problem from the firing order by finding the paired cylinders.

Any component thats fused through the fuse block can be draw tested there.