A coil pack is a pack of ignition coil generally located in the top area under the hood, above the engine. Part of a car's ignition system, it is typically box-shaped in appearance, and amplifies the voltage sent to individual spark plugs. There are several ways to determine if your car has a weak coil pack.
Car is Misfiring
One of the earliest signs of determining a weak coil pack is engine misfires, also known as "hard starting." A misfiring engine will cause the car to sound choppy or shake, and can make it feel as if the vehicle is stalling. Also, a decrease in power can be attributed to a failing coil, which will cause a low-spark intensity.
Another way to determine a weak coil pack is just by observing if there is any damage or cracks. Generally, the coil pack is box-shaped, with corresponding coils to fire each cylinder. While a car is running, coils are exposed to heat. A cracked housing on a coil needs to be immediately replaced, as it can expose it to moisture. Look for loose or damaged coil connectors.
Check Engine Light
A "check engine" light can mean numerous things, one of which is a weak or defective coil pack. If the light comes on, and you can determine that the spark plugs are fine, the coil pack probably needs replacing.
A sure sign of a weak coil is that it produces a weak yellowish or orange-coloured spark. A coil, when healthy, should produced a clear and bright bluish-white spark--clearly visible in the daylight--accompanied by a distinct "cracking" sound that is loud. Test the coil spark using a fully charged battery, but only do so if you are aware of the dangers involved, and have a helper to turn the engine. It is probably best to consult with a mechanic or auto expert for spark testing, as this involves holding the spark plug wire with pliers.
OBD II Tester
You can test coil packs using an OBD II diagnostic scanning tool, or code scanner. It can read error codes with the vehicle's computer, and can diagnose the functionality of the coil pack. All cars built since January 1996 have OBD-II systems. They can be purchased at an auto retailer. You could also contact an auto repair shop or the nearest car dealer with on-site mechanics.