Diagnose a bad coil pack by using an ohmmeter to compare the vehicle's readings to standard values for the vehicle. Use a scan tool to determine which cylinders are misfiring as a result of the defective ignition coil. Water can also help to diagnose a bad coil. Use a spray bottle to mist water over the coil pack to determine which areas of the engine are misfiring.
A bad coil pack does not properly help ignite fuel in the cylinders by providing the spark plugs with electricity. Diagnosing a bad coil pack requires the use of a 12-volt test light or an ohmmeter to connect to the vehicle’s battery. Diagnosing this issue also requires noticing specific symptoms, such as an activated check-engine light, rough idle, reduced power, excessively loud engine and smoke exiting the exhaust.
Early signs of a defective coil pack include a drop in RPMs, decreased engine power, excess noise and rough idle. Techniques for diagnosing a bad coil pack are essential to repairing the problem. Start by locating the coil pack to get access to the problem. The coil pack is located next to the oil dipstick and is generally found by following the spark plug wires.