A car's ignition coil provides a spark to its engine's spark plugs. It is essentially a small transformer that converts energy from fuel combustion into a high-voltage form that is usable by the spark plugs. Older vehicles typically have a single ignition coil, while newer vehicles typically supply each spark plug with its own individual coil. If a coil fails due to overheating or physical breakage, then it will cause diminished performance in newer vehicles and will cause the engine to stop running entirely in older single-coil vehicles.
Engine Stops Running
If you have an older vehicle that possesses only a single ignition coil, then there is only one symptom associated with ignition coil failure. Once the ignition coil fails it will be impossible to provide energy to your engine's spark plugs and the car will cease to run. If your car stops running entirely then you can consult your car's user manual to determine whether it is a single-ignition coil car; if it is a newer car with multiple ignition coils then your engine's total failure is being caused by a different problem.
Decreased Gas Mileage
If you own a newer car with multiple ignition coils and one of your ignition coils fails, you should expect to observe an overall decrease in fuel efficiency. The decrease should be much more subtle under conditions in which you drive at a slow rate of speed and do not accelerate rapidly. In such conditions the engine is not running at full capacity and the faulty coil will have relatively little impact on performance. Your car's fuel efficiency will take a much greater hit under highway driving conditions or when accelerating quickly, since all power that is being transmitted to the faulty coil will dissipate and be lost from the system.
Newer cars will also experience a lack of power when an ignition coil fails. Again, this lack of power will only be evident under high-performance driving conditions. It is unlikely that you will see much of a difference in acceleration on a relaxed Sunday drive through town, but you will probably notice the diminished engine power when you are accelerating to merge onto the highway.
A faulty engine coil may also slightly increase the amount of pollution that is emitted by your vehicle. These increased emissions may be due to incomplete combustion, in which a portion of the energy created by burnt fuel is lost from the system and emitted through your exhaust. Increased emission levels may also be observed if you have a partially functional ignition coil that misfires erratically.
Check Engine Light
In new vehicles, failure of an emission coil is likely to trigger your car's check engine light. If your coil is degrading gradually then your car's on-board diagnostic system may even detect performance decreases associated with ignition coil problems before the problems reach the stage at which you can notice any problems yourself.
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