The ignition coil takes power from the battery and relays it to the spark plugs, which burns fuel, which makes your car run. If one or more of the ignition coils fail, your car's ability to run efficiently will be greatly hampered. The degree to which, and the number of ignition coils that are failing, as well as the total number of ignition coils the car runs on, determine the severity of the car's symptoms.
One of the first symptoms that a coil is no longer working as it should is a decline in your normal gas mileage. This is because it takes more fuel for the car to run when less power is reaching the spark plugs. So, when your ignition coil begins to fail and becomes less able to transfer power, your car requires more fuel to run, and your fuel economy suffers.
Also the result of increased, and less efficient, fuel use is a noticeable change in your exhaust. It may become black and smell of gasoline, rather than smelling like normal exhaust fumes.
If you do not catch the earliest symptoms of a failing ignition coil, your car may begin to experience serious backfires. This happens when unused fuel is emitted through the car’s exhaust, creating a small explosion. Backfires can occur either in the car’s manifold or in the exhaust pipe. Typically, backfires caused by ignition coil issues take place in the latter. If the problem is not corrected, these backfires can cause serious damage to your exhaust system, which will require costly repairs, which would otherwise be unnecessary.
Hard to Start
Because ignition coil failure means that a spark plug is no longer receiving an appropriate amount of charge, cars that are suffering from this issue are typically harder to start. This may be especially noticeable when the car is cold, or when the humidity level is high.
A series of misfires will often occur when ignition coils fail, resulting in your car running rough. When travelling at highway speeds, this may feel like a jerking sensation or a random, but repeated loss of power. At a traffic light, or when idling for any reason, this feels like a vibration.
It is essential that your car continue receiving sparks to stay running. Cars that have ignition coil issues may stall, especially when idling, like at a stoplight or when left running while parked. This may become especially noticeable after the engine has been running a while and has “warmed up.”
Failure to Start
A car whose ignition coil(s) completely fail will not start at all. In this case, jump starting will prove ineffective.
The most common way to mechanically diagnose a failing ignition coil is to check the wires that attach to the spark plugs for transfer of power. If, when tested, there is no “fire” in the wire, which would indicate no power was reaching the plugs, then your ignition coil is not appropriately converting and transferring energy from the battery to the spark plugs.