The ignition coil converts the low battery voltage to the many thousands of volts needed to create the spark that ignites the fuel and air mixture with the spark plugs. Driving a car with a bad ignition coil causes a cylinder misfire putting added strain on the engine, and the engine will run noticeably rougher with less power than normal. It is also possible to damage the catalytic converter or oxygen sensors because unspent fuel may make it into the exhaust system. You should think about replacing your ignition coil if you notice one or more of the following problems: - The “check engine” or warning light comes on. - Your engine stalls. - Your engine has a rough idle. - Your engine backfires. - It is hard to start your car. - Your vehicle consumes fuel at a faster rate. - Your vehicle has increased emissions.