Do you mean a "driver" circuit? Drivers circuits are transistorized switching circuits for ignition coils, injectors, relays, and other devices controlled by the Engine Computer. They rarely fail. It's more common to find cut wires, corroded splices, or corroded connector terminals between the device and the computer. Most of the time a visual inspection will identify them. An ohm meter is used to measure the continuity of wires. Experienced mechanics may also use voltage drop measurements but that's pretty involved and hard to do in circuits that have pulsing voltages like for ignition coils and injectors.

Answered by caradiodoc (expert)
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