How do you know which coil is faulty?

To identify a faulty coil, you can perform a resistance test using a multimeter. Measure the resistance across the coil terminals; a significantly higher or lower reading than the manufacturer's specifications indicates a problem. Additionally, visual inspection for physical damage, corrosion, or overheating signs can help determine if the coil is faulty. If the coil is part of a system, monitoring performance issues like misfires or poor ignition can also signal a fault.