Symptoms Of Bad Spark Plugs & Replacement Cost

Symptoms Of Bad Spark Plugs & Replacement Cost

Spark plugs are an important component of any gasoline engine because they ignite the fuel in the engine’s internal combustion chamber and keep the engine running successfully.

But sometimes, after a long time without care, the spark plugs get damaged, due to which many spark plug-related problems are seen in the engine.

However, it does not cost much to replace a bad spark plug in the engine, and the spark plug is a small component of the engine. But if the spark plugs of the engine get damaged, then the performance of the engine can be badly affected.

In this post, we will learn in detail about the symptoms of a bad spark plug and its replacement cost.

Symptoms of Bad Spark Plugs

Common symptoms of bad spark plugs include the check engine light coming on, engine rough idling, trouble starting, acceleration hesitation, engine vibration, poor power performance, poor fuel economy, or a smoke problem.

1. Engine Misfire

engine misfire

The engine of a vehicle consists of many parts and each part does a different job. The principle of operation of petrol and gasoline engines is based on the timing of air, fuel and spark current.

When the vehicle’s ECU sprays a mixture of fuel and air into the engine’s internal combustion chamber at the correct timing according to the position of the piston.

The ECU then generates spark current for the combustion of the fuel mixture through the spark plug, due to which the engine starts after successfully completing the internal combustion process.

But a bad spark plug output is unable to draw spark current or intermittently supplies spark current, due to which the engine starts misfiring and the running engine starts jerking. A bad spark plug can also be a major cause of engine misfiring.

2. Engine Rough Starting


Whenever you try to start gasoline and diesel engines, due to a bad spark plug, the combustion chamber of the engine does not spark current with the proper amount and timing, which causes the engine to start rough. Sometimes the engine does not start, even after trying for a long time.

A bad spark plug can also be one of the main symptoms of a rough start. However, these symptoms can also occur if the spark plug has not supplied the input power supply.

3. Engine Vibration

A faulty spark plug can sometimes result in intermittent sparking of current in the combustion chamber of the engine, due to incorrect timing of the piston and fuel mixture spray position, which causes the engine to vibrate or the engine starts jerking when the engine is started.

A bad spark can also be one of the other signs that an engine is vibrating. If this happens, a professional vehicle mechanic may recommend getting the vehicle inspected because the vibration of the engine can be harmful to the engine due to prolonged running.

4. Engine Low Performance

engine low performance

Engine is one of the power supply parts in vehicles. The vehicle gets power from the engine, due to which it gets speed or else the other functions and parts of the vehicle are operated by the engine itself. But the performance power of the engine decreases due to poor spark plugs.

Due to this, the engine starts to provide less power for vehicle speed and other functions when there is any kind of load on it. Then the engine starts misfiring. A bad spark plug is not able to spark the right amount of current.

The vehicle’s overall efficiency decreases when the spark plugs are unable to generate the correct quantity of power. They effectively cause much of the power to be wasted since they are unable to properly ignite the fuel mixture to produce an adequate level of power.

5. The Check Engine Light On

check engine light

Engine performance can suddenly change when spark plugs are faulty or are not working properly. When this happens, the check engine light in the car’s cluster meter may be constantly on or the check engine light may start blinging. Because without service or replacement for a long time, spark plugs start getting weak.

And the working power of the spark plug starts decreasing. With the spark plugs inside, the engine gets too hot and starts idling.

Due to this, there is an obstacle to passing the current. This causes the check engine light to illuminate. At this time, you will need to make an appointment for the maintenance of your car, even if the engine is not causing any problems or making any noise.

6. Low Fuel Mileage

high fuel consumption

Bad spark plugs can reduce your car’s fuel mileage by up to 30%. and due to a bad spark plug, there is the possibility of other types of disorders in the engine. The excessive fuel consumption of your car in running 1000 miles to 2000 miles can be more than the cost of new spark plugs.

Because bad or weak spark plugs do not completely ignite the fuel in the internal combustion chamber of the engine, the half-burned fuel comes out of the exhaust pipe in the form of raw smoke.

And the performance of the engine starts decreasing. Therefore, bad or weak spark plugs can severely affect the fuel mileage of the engine to a great extent.

7. Acceleration Issue


When we accelerate an engine. Then the RPM of the engine increases rapidly and the power of the engine also increases, causing the vehicle’s speed to increase and, for other tasks, the engine to apply more force but with bad or weak spark plugs when the engine is given acceleration.

The engine then misfires due to not getting the right amount of spark to ignite the fuel mixture or starts jerking loudly. Also, the engine starts making more noise.

And also, when the engine is accelerated, the RPM of the engine does not increase by the right amount. Engine acceleration problems can also be a symptom of a bad spark plug.

8. Black Smoke Issue

black smoke from exhaust

Black smoke coming from the exhaust pipe of the engine can also be a major symptom of bad or weak spark plugs. However, there can be many other reasons for black smoke coming from the exhaust pipe of the engine.

But among the symptoms and causes of black smoke from the exhaust pipe of the engine, bad or weak spark plugs can also be a symptom. Because a bad or weak spark plug can cause the fuel mixture to ignite in the right amount. Due to this, the unburned fuel comes out of the exhaust pipe in the form of black smoke.

Spark Plug Replacement Cost (2023)

The cost of a spark plug replacement can usually range between $5 and $20. However, these costs can be high in some engine models that require you to replace 4 to 16 spark plugs.

The cost of spark plug replacement depends on several factors. Such as the number of cylinders in the engine, the quality of the spark plug, the type of spark plug, etc.

  • Copper Spark Plug 1 Piece – $5 To $7
  • Platinum Spark Plug 1 Piece – $6 To $19
  • Double Platinum Spark Plug 1 Piece –  $6 to $20
  • Iridium Spark Plug 1 Piece –  $8 to $20

There are 4 spark plugs in a 4-cylinder engine. But before changing the spark plug, it is mandatory to inspect, how many spark plugs are there or if all the spark plugs are bad and weak. This can make the spark plug replacement cost more economical.

The labor cost of a spark plug replacement by a professional auto mechanic can range from $20 to $50. if you can replace the spark plug yourself. So, you can save on mechanic labor charges. because it is very easy to replace the spark plugs of most vehicles.


Q. – How do I know when my spark plugs are bad?

A. – Whenever the spark plugs in your vehicle’s engine are worn or damaged, you have several symptoms indicating bad spark plugs. On the basis of which you can identify the bad position of the spark plug. such as

  • Engine not starting
  • Engine starting after long rounds of crank
  • Rough idle of the engine
  • Acceleration problem
  • Engine misfiring
  • Low fuel mileage
  • Decreased engine power and performance
  • Engine backfire

Q. – What does a bad spark plug feel like?

A. – A rattling, pinging, or knocking sound can be heard in the engine when the spark plugs of the engine go bad. Along with this, vibrations and tremors can also be felt in the engine.

Q. – How often do spark plugs need to be changed?

A. – An engine’s spark plug is designed to be a component used for a mileage period. Usually vehicle manufacturers and spark plug manufacturers recommend replacement between 80000 miles to 100000 miles. However, the lifespan of spark plugs also depends on their maintenance and timely service.

Q. – Can I drive with bad spark plugs?

A . No, it is not recommended to drive the vehicle for extended periods of time with bad spark plugs in the engine. Because of a bad spark plug, the engine gives off vibrations and jerks, due to which the internal parts of the engine may be damaged and the worst effect is on the engine mount.

Q. – Is it ok to replace just one spark plug?

A. – Yes, if one of the spark plugs in the engine is bad. Then you can replace that one spark plug. For this, you have to diagnose the bad plug. For this, you can take advantage of a digital OBD scanner.

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