Bad Ignition Coil Symptoms: Diagnose, Test and Replacement Cost

This guide will help you understand what an engine coil is, how to test and diagnose bad ignition coil symptoms. We will also cover how much ignition coil replacement costs in case your test ignition coil needs to be replaced.

What is an ignition coil?

The ignition coil is an engine component that's needed to create an engine spark. Any kind of problem can lead to poor engine performance, which can affect your safety on the road. Experts call an ignition coil as a mini electrical transformer as they use a low 12-volt current in car batteries and use these to create a higher voltage that's necessary to ignite engine fuel and start your car's engine.

Every spark plug in your car has an ignition coil, and this may be connected to the spark plug using wires, or it may be found over the spark plug, so there's no need to use wires. Your car must have strong, efficient ignition coils to produce enough electricity to create an electric spark to ignite the fuel. Problems with ignition coil systems can lead to poor fuel consumption or problems like engine misfires.

Sometimes, bad ignition coils can be due to other problems like a low or high voltage battery. And aside from poorly-performing ignition coils, there are many other issues that a troublesome car battery can bring. All these can go away when you install a new battery.

This is especially true with batteries found in older cars as these breakdowns due to the battery's age. Take note that a good battery should at least read 12.65 volts when it's completely charged. A reading lower than this could mean that there are problems with the car battery.

What are bad ignition coil symptoms?

The following are common symptoms of ignition coil issues:

1. There is backfiring

Backfiring is one of the most common early symptoms of a bad ignition coil. Backfiring happens when any unused fuel found inside the combustion cylinder moves through the exhaust. There is backfiring when there is the presence of black smoke coming from the exhaust pipe.

If you smell gasoline from the black smoke, these are early signs of ignition coil problems. If you notice these symptoms, take your car to a repair center right away. Usually, overlooked bad ignition coil symptoms like backfiring can lead to severe and expensive engine repairs.

Related article: quick fix for white exhaust smoke

2. Engine misfiring

Engine misfiring is a late sign of failing ignition coils. If you start an engine with failed ignition coils, you will hear misfires sounding like coughing and sputtering.

When you're driving fast, misfires can be experienced as spitting and jerking. An idling car at a stoplight or stop sign may be vibrating very strongly when it has bad ignition coils. 

3. Poor fuel consumption

If you notice that your car is consuming more fuel than before, then this could be a sign of a bad ignition coil.

4. Engine idling, stalling, jerking, and overall poor power

If you feel that your engine is poorly performing, missing power when you accelerate, then these may be due to an ignition coil problem. If your engine is idling, jerking, or there is some hesitation as you accelerate, then these could also be signs of ignition coil issues.

5. The check engine light is lit with DTC code

Suppose you notice that the check engine light is always lit or there's an engine code PO351 (this means Ignition Coil – Primary/Secondary Circuit Malfunction) on your electronic dashboard after performing car diagnostics. You may contact a car repair shop to check for this error. Aslo, with a faulty cruise control, the check engine light flashing shows the symptom of a bad speed sensor.

6. Hard to start the engine

If you find that your engine is very hard to start, then having bad ignition coils can be the reason. If your vehicle has a single coil, then a malfunction with this coil can lead to a stalling engine. The engine is cranking with no sparks in the cylinders.

Tip: The body control module ( ignition control module ) in automotive generally controls the ignition.

How to test the ignition coil

You can safely test your ignition coils by following these steps:

  1. Turn your engine off to avoid any accidents.
  2. Open your car's hood to check the ignition coil. Because the location of the ignition coil varies in every car make and model, check your car's manual or look for significant resources online.
  3. Disconnect one of the wires on the spark plug and remove the spark plug with a socket.
  4. Re-seat the wire on the spark plug.
  5. Place the threaded part of the spark plug to any exposed metallic part of your car.
  6. Remove your car's fuel pump relay or the fuse.
  7. Ask someone to turn the car's ignition key.
  8. You can tell that the ignition coil is working if you notice a blue spark on the spark plug head when it touches the metal car part.

If you're not comfortable with checking or testing your ignition coils, have a professional do this for you.

You may want to know oil on spark plugs symptoms.

How much does it cost to ignition coils?

The ignition coil cost will always depend on the model and make of your vehicle. The cost of cheap coils are $70, but there are expensive coils that cost around $300. Labor costs for ignition coil replacement also vary from $50 to $100 for every hour of work.

Contact a local car repair shop to ask for an estimated cost of ignition coil replacement. Check if your car insurance will cover bad ignition coil problems or ignition coil replacements. 

Can you prevent a bad ignition coil problem?

Prevent bad ignition coil issues by regularly inspecting your car's ignition coils and spark plugs for any type of erosion and wear and tear. Take special attention that the wires are in good condition, the prime housing is intact, and there is no oil leaking. Ask your car manufacturer how to perform regular maintenance on your vehicle.

Also, never take for granted early bad ignition coil symptoms, especially backfiring and engine misfires, because these are engine problems that can lead to more complicated issues. Have your car checked ASAP?

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