bad spark plug

Spark Plugs Symptoms, Function & Replacement Cost

In Troubleshooting by Josh SLeave a Comment

bad spark plug

Have you noticed that your engine is not running as it should and the engine light showing up sometimes? It may be caused by a bad or failing spark plug. The spark plug is a very important component of your engine and it can cause a lot of strange symptoms and even damages to your engine.

In this article, you will learn what a spark plug is and which symptoms you should look for when you are troubleshooting your engine for a bad spark plug. You will also get information on the cost and how to replace the spark plug.


What is a spark plug?

The spark plug is the component of your engine that is igniting the cylinder fuel/air mixture. The fuel injectors are injecting fuel into the combustion chamber and then the piston is compressing the air-fuel mixture. At this event, it’s time for the spark plug to ignite the fuel mixture to push the pistons down. If there is a problem with the spark, the air-fuel mixture won’t get ignited and it will blow out unburned through your exhaust pipe. That event is called a Misfire.

You do maybe understand the importance of the spark plugs and why it’s a very important part of the engine to keep in good shape now.


Spark Plugs Symptoms

Because of the importance of the spark plugs in the engine, there could be a lot of different symptoms on a car engine when you have bad spark plugs. I work as a car technician, and I will list the most common symptoms from a car with bad spark plugs.

The most common symptoms of Bad spark plugs are rough idle, rough acceleration, misfires, hard starting condition, poor fuel economy and you will probably experience a Check engine warning light on your dashboard. The Spark plugs are an important part of the engine and it could cause a lot of problems.

Bad Spark PlugsEngine Misfires

Check engine light

Rough Idle

Slow/Rough Acceleration

Hard Starting condition

Poor fuel economy
Read your trouble code memory with an OBD2 scanner and check for any trouble codes of misfires.

Replace Spark plugs

1. Engine Misfires

The most common symptom of bad spark plugs is that you will feel misfires from your engine. Misfires can cause the engine to run rough at idle, rough at acceleration and fuel economy changes. The check engine light will probably also show up after a while. You can feel the misfires as the engine will probably running rough and the engine sound may not be as it used to be.

2. Check engine light

When the engine is misfiring, the engine control unit will often recognize the misfire and find out which the misfire occurs on and store it as a trouble code and lighten up the check engine light. That’s why it is a very good idea to read the trouble codes with an OBD2 scanner when you are troubleshooting your car.

3. Rough Idle

When a misfire occurs due to bad spark plugs, the unburned fuel may trick the O2 sensors. The O2 sensor will then regulate the fuel mixture and you may get a lean mixture. This can cause rough idle and even more misfires on other cylinders.

4. Slow/Rough Acceleration

Due to the O2 sensors mentioned before, you can also get a slow and rough acceleration from misfires and lean mixture. You will often feel this at hard accelerations.

5. Hard Starting condition

Because of the importance of a good spark in the starting moment of an engine, bad spark plugs may cause the engine to not start at all or give you a very long cranking time before it will start.

6. Poor fuel economy

Because of the misfires and the O2 sensor regulations, you can get a poor fuel economy from bad spark plugs. This will most likely not be a dramatic difference, but it can be because a misfire can cause a lot of problems for the fuel mixture.


Will my engine get damaged if I keep driving with my bad spark plug?

Yes, it will at some point. Bad spark plugs will cause misfires and misfires can cause several problems in the engine. One important thing to take notice is if you have a catalytic converter. The misfires lead to unburned fuel that will blow out through your exhaust pipe. If you have a catalytic converter fitted in your car, the unburned fuel will get stuck there and will get ignited in the catalytic converter due to the high temperature of it. It can cause your catalytic converter to get damaged, and a new catalytic converter can be very expensive. Misfires can also cause a lot of mixture problems with your engine and damage other parts.


How long do spark plugs last?

The time a spark plug will last, depends on a lot of things. First, you have to check if you have platinum/iridium spark plugs or regular. Platinum/iridium plugs do usually last longer.

You should always check with your authorized dealer or check in your service manual when it’s time to replace the spark plugs. A regular spark plug will often last 60.000km’s (37.000 miles) while a platinum/iridium plug will last around 90.000 km’s (56.000 miles). But you should always make sure what the scheduled time is for a spark plug replacement for your car model and engine.


Spark Plug Replacement

The procedure of a spark plug replacement can differ a lot depending on what car model and engine you have. Some are very easy and some engines take a lot more skill to do it. Some V-engines can be very hard to reach the spark plugs but the principle is the same.

  1. Remove the parts over the ignition cables/ignition coils
  2. Remove the ignition coils/ignition cables
  3. Remove the spark plug(Often a 16mm sized socked, depending on the car model)
  4. Check the length and make sure you got the right spark plugs(Too long spark plug threads can cause serious damaged to your pistons)
  5. Spray a little bit of lube at the threads on the spark plug and tighten it with the right torque(Often ~15-20 nm)
  6. Refit the ignition coils/cables and all the other required parts.


Replacement cost

The cost of your spark plugs replacement is depending on how many cylinders you have, what type of spark plugs that are required for your car engine and how hard it is to replace it. If you are going to replace the spark plugs yourself, it will not cost that much.

A regular spark plug will cost 1-5$ each and an iridium/platinum spark plug can cost 5-15$ each. If you want to find good spark plugs you can find them on Amazon on the links down below. Always make sure that the spark plugs will fit your vehicle before buying.

Denso Regular Spark Plugs

NGK 5464 BKR5EIX-11 Iridium IX Spark Plug, Pack of 4

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