An ignition coil is a transformer. Its job is to boost the car battery's low voltage (12V) to a very high voltage (over 20,000V) needed to create a spark at the spark plug.

It works in two steps:

Charging (Store Energy): The car's computer sends a 12-volt current into the primary winding (a coil of thicker wire) inside the ignition coil. This creates a magnetic field around it, storing energy.

Discharging (Release Energy): At the exact right moment, the computer cuts off the 12-volt current. The magnetic field collapses very quickly. This collapsing field induces a very high-voltage surge in the secondary winding (a coil of much thinner wire with thousands more turns). The high-voltage surge travels to the spark plug and jumps the gap as a spark, igniting the fuel-air mixture in the engine cylinder.

An ignition coil is constructed from copper wire and an iron core to perform a specific energy conversion task. The process begins when the primary winding (made of thicker copper wire) receives a low-voltage current, building a magnetic field in the iron core. Instantly after, the current is cut off, causing the magnetic field to collapse rapidly. This sudden change induces a powerful electrical pulse in the secondary winding (made of much finer, thousands-of-turns copper wire), generating the high-voltage spark required for combustion.

Engine Misfire: The engine jerks, stutters, or stumbles, especially when accelerating. You might feel a vibration.

Check Engine Light: This is a very common sign. The car's computer detects the misfire and turns on the warning light.

Poor Fuel Economy: Incomplete combustion leads to wasted fuel.

Rough Idling: The engine shakes and runs unevenly when stopped.

Difficulty Starting: The car may struggle to start, especially if multiple coils are weak.

It's not recommended. Driving with a misfire can damage other components, like the catalytic converter (which is very expensive to replace), and it significantly increases emissions. It's best to get it fixed as soon as possible.

There's no set mileage, but they typically last between 80,000 to 100,000 miles (130,000 to 160,000 km). However, they can fail earlier due to heat, vibration, or moisture.

It's not always necessary, but it can be a good idea, especially if your mileage is high. If one coil fails, others are likely near the end of their life. Replacing them in sets can prevent another breakdown and labor cost soon after.

They work together as a team:
Ignition Coil: Creates the high-voltage electricity.
Spark Plug: Receives that high voltage and creates the actual spark to ignite the fuel.

Not directly. A bad coil won't typically drain the battery while the car is off. However, a severely misfiring engine can put extra strain on the charging system, which might indirectly affect the battery over time.