Although there are a number of suggested tests for the ignition coil while it is still attached to a car, the only accurate way of testing a coil is on a bench using a multimeter in the resistance setting. The ignition coil has two coils of wires, called windings. The meter quickly displays if either winding is bad.
The primary winding collects the electricity needed to create the spark and the second sends it to the distributor. If either is bad, the coil is failing. Using a multimeter on a disconnected coil provides the data needed to determine its health. Start by checking the resistance between the coil's side terminals. With all but a few coils, the meter should show 0.75 to 0.81 ohms of resistance. Next, check the resistance between the center terminal and either side terminal.
A good coil reads between 10,000 and 11,000 ohms. Replace the coil if there is any significant deviation from these numbers. Cars with a bad coil often run or idle rough. During cold weather, starting the car becomes difficult. As the coil continues to deteriorate, the vehicle eventually fails to start. Replacing a defective coil helps the vehicle to start quickly and run smoothly, increasing its fuel efficiency.