If your car won't start or it hesitates when you crank it, you might have a bad ignition coil. Testing an ignition coil on most cars is pretty easy. No special tools are required, and you don't even have to remove the coil. Just remember to be careful, because the amount of electricity generated by your ignition system can be dangerous.
What the Coil Does
An ignition coil is a kind of high-voltage transformer. It converts the power from your car's 12-volt battery into the thousand or so volts that the spark plugs need in order to fire and then ignite the engine's air-fuel mixture. Although manufactured to last, ignition coils are not fail-safe. Over time, heat, vibration from the road, and even degraded insulation will necessitate that you change out this part.
You can easily test the coil on your car yourself, either while it's still on the car or after you've removed it. Just take a few precautions first. Make sure you're wearing a pair of safety glasses and old clothing with no loose sleeves or shirttails that can get caught in the engine or elsewhere on the vehicle. Also be aware that your car's engine is an electrical system, so be cautious when you are working on a running engine and avoid those parts and processes that can deliver a shock—or worse.
Testing the Coil
You don't have to remove the coil in order to test it. It can be done by checking the spark plugs. For this test, you'll need a pair of safety goggles, some insulated pliers, a spark plug socket tool, and a helper. The test can be done in two easy steps:
First, turn off your car's engine. Then consult your owner's manual to locate the wire that attaches to one of your spark plugs. Remove the wire and use a spark plug socket tool to remove the spark plug from its socket. Be careful not to let anything drop into the empty spark plug hole.
Next, reattach the spark plug wire. Hold the spark plug with a pair of insulated pliers and touch the threaded bare end of the spark plug to a grounding surface. Pretty much any exposed metal, including the engine itself, will do. Have your helper crank the engine while you look for a bright blue spark to jump across the spark plug gap. If you see a nice, bright spark (clearly visible in daylight) your coil is doing its job.
If your coil is already off the car, or if there was no spark or the spark was weak, then move on to bench test your coil. This test will tell you without question whether or not you have a bad ignition coil.