Engine start needs spark plug fire which requires high electricity around 20,000 to 35,000 volts to initiate. Ignition coils are like transformers to convert voltage from 12 volts to this high voltage pulse to fire the spark plugs. The number of spark plug varies on car models and it's a trend that number is increasing in newer models depends on the cylinder number. Coil-on-plug type is getting more and more common which means each spark plug has its partner ignition coil to collaborate to start the engine.
Symptoms of a Bad Ignition Coil & When to replace Ignition Coils
#1 Engine Misfires A weak ignition coil will cause "NO spark" situation as it is unable to deliver enough power to spark plug. When you hear the sounds like "coughing" or "sputtering" while you start the engine, it could be the sign that ignition coil has something wrong.
#2 Hard start or No start
#3 Power Losing or Poor Acceleration
#4 Car stalling, jerking, and rough idling
#5 Poor Fuel Economy If the fuel economy is noticeably decreasing, better check out the ignition coils as the failure is occurring.
No. A faulty coil may cause the above problems but will not lead an immediate safety issue. However, malfunction coils will let incomplete burning fuel enter and damage the catalytic converter from overheating. So better to let mechanic to have thorough check for your car when you experience above problems.