Bad Spark Plug Symptoms And How To Fix It | Perfect Guide

If you start experiencing issues like rough idling, decreased fuel efficiency, or even difficulty starting your vehicle, it may be time to examine one crucial part, like a spark plug. As a vital component in any combustion engine, faulty or bad spark plug symptoms can lead to various problems that can negatively impact your car’s performance.

Understanding the symptoms of bad spark plugs is vital as it allows you to identify potential issues early on and take appropriate action before more severe problems arise. We will explore some common signs that indicate your vehicle’s spark plugs might be due for replacement or repair, helping you maintain optimal performance and prevent further damage down the road.

10 Bad Spark Plug Symptoms

Increased Fuel Consumption

A bad spark plug can wreak havoc on your vehicle’s fuel efficiency, causing a significant increase in fuel consumption. One of the first signs of bad spark plugs is poor ignition. When the spark plug fails to ignite the air and fuel mixture properly, it leads to incomplete combustion. More fuel is required to compensate for this lack of combustion efficiency, leading to increased fuel consumption.

Another symptom of a bad spark plug that can contribute to higher fuel usage is misfiring. A misfiring spark plug occurs when it fails to generate enough or produces sparks incorrectly. This disruption in the ignition process not only affects engine performance but also leads to wasted fuel. The unburned air and fuel mixture caused by misfires increases carbon emissions and gasoline consumption.

Harder Time Starting

When you turn the key in your car’s ignition, and it takes longer than usual for the engine to start, it could be a sign of a terrible spark plug. If the spark plug is faulty or worn out, it may not produce enough sparks or fail to ignite altogether.

Before jumping to conclusions about your spark plugs, checking your battery first is always wise. A weak battery can also cause difficulties in starting your car as it may need to provide more power for the necessary electrical components.

To determine whether an issue with starting lies with your battery or spark plugs, inspect your battery terminals for any signs of corrosion or loose connections.

Loss of Acceleration Power

When your vehicle is experiencing a loss of acceleration power, it could indicate a faulty spark plug. If one or more spark plugs are not performing optimally, it can decrease acceleration power and overall engine performance.

One common symptom of an unfit spark plug affecting acceleration is sluggishness during takeoff. When you press down on the gas pedal to accelerate, you may notice that your vehicle has difficulty gaining speed quickly.

This lack of responsiveness can be frustrating and potentially dangerous when merging onto highways or passing other vehicles. If this issue persists, it is essential to have your spark plugs inspected and replaced, if necessary, by a qualified mechanic.

Your Engine Misfires

A clear sign that indicates an undesirable spark plug is an engine misfire or stuttering during acceleration. When a spark plug fails to ignite the air-fuel mixture properly, it disrupts the combustion process, resulting in incomplete fuel burning and reduced power output.

This loss of power often causes noticeable jerking or hesitation when accelerating, making your driving experience less smooth and efficient. To decrease performance, you may also notice poor fuel efficiency due to this incomplete combustion.

Your Engine Backfires

Engine backfiring is one of the most common symptoms of a faulty spark plug. A spark plug is responsible for igniting the air-fuel mixture inside the combustion chamber, but when it fails to do its job effectively, unburned fuel can build up in the exhaust system and ignite there instead.

A backfire occurs when this explosion happens outside of the combustion chamber. It can be startling and even dangerous if addressed. Along with the loud noise and sudden jolts, you may also notice black smoke from your exhaust pipe and a foul smell. These are clear indicators that something is wrong with your spark plugs, and immediate attention is necessary to prevent further damage to your engine.

Car “Vibrates” When Idle

A symptom of a poor spark plug is engine vibration when your car is at a standstill. When the spark plug isn’t functioning correctly, it can cause uneven firing in the cylinders. This can result in rough idling and noticeable vibrations throughout the vehicle. If you feel an unusual shaking or trembling sensation while waiting at stoplights or while parked, it’s likely time to inspect and replace your spark plugs.

Engine Surges

A bad spark plug can lead to a variety of issues, one of which is engine surges. Engine surging refers to sudden and irregular increases in RPM (revolutions per minute), causing the engine to lurch forward unexpectedly. This can be alarming and dangerous, especially when driving at high speeds or in heavy traffic.

Engine surges are a faulty spark plug; when a spark plug starts to fail, it may not ignite the air-fuel mixture efficiently, resulting in an uneven combustion process. The engine experiences disruptions in its normal power delivery rhythm, leading to erratic surges. These surges can occur during acceleration or even while idling, making it difficult for drivers to control their vehicles.

The “Check Engine” Light Turns On

If you notice that your check engine light has turned on, it may indicate a negative spark plug. The check engine light is a vital warning system in your vehicle, designed to alert you to potential issues with the engine. While there can be several reasons why this light turns on, one common culprit is a faulty spark plug.

If you notice this illuminating in your dashboard, it must be checked out promptly to prevent further damage or potential breakdowns. You may need more than the check engine light to give you an accurate diagnosis of a bad spark plug.

A vibrating car when driving is another common symptom that accompanies this issue. A poorly functioning spark plug can cause irregular combustion in the engine cylinders, leading to noticeable vibrations throughout the vehicle while in motion.

Engine Hesitation

Engine hesitation occurs when there is a delay or lack of response from the engine after pressing the gas pedal. You may notice that your car takes longer or feels sluggish during acceleration. This can be frustrating and dangerous, especially when quick maneuvering is necessary.

The electrodes in the spark plug can become eroded over time or covered in carbon deposits, resulting in a weak spark. This weak spark can lead to misfires or incomplete combustion, which then causes the engine to hesitate.

Engine Knocking

This distinct noise occurs when the air-fuel mixture inside the combustion chamber ignites unevenly, resulting in a series of small explosions rather than one smooth combustion process. The knocking sound can range from a light tapping to a loud hammering sound, typically becoming more noticeable as you accelerate.

Ignoring the engine knocking due to a faulty spark plug could decrease fuel efficiency and power loss. As the spark plug deteriorates, you may experience rough idling or stalling while at stoplights or during low-speed maneuvers.

Tips for Maintaining Your Spark Plugs

It might be time to check on your spark plugs. These small but vital components are crucial in starting and running your vehicle. To ensure they function at their best, follow these tips for maintaining your spark plugs.

Inspect Your Engine Wires

Inspecting the engine wires is a crucial step in maintaining your spark plugs. Start by opening the hood and finding good lighting to get a clear view of the wires. Look for any signs of wear or damage, such as cracks, fraying, or burn marks. These issues can lead to poor performance and misfires in your engine. If you notice any problems with the wires, it’s essential to replace them promptly.

Check the connections between the spark plug wires and the spark plugs themselves. These connections can become loose or corroded, affecting their ability to deliver electricity efficiently. Remove each wire from its spark plug and inspect both ends for any dirt or corrosion buildup. If needed, clean these areas using a wire brush or a soft cloth dampened with an electrical contact cleaner.

Run the Engine

Maintaining your spark plugs is essential to keeping your engine running smoothly. When you start your engine, pay attention to whining or straining noises from under the hood. These noises could indicate a high volt leakage in the spark plug system. By catching this issue early on, you can prevent damage to other engine components.

To listening to strange noises, another tip for maintaining your spark plugs is to inspect them regularly visually. After running the engine, open the hood and locate the spark plugs. Check each individually and look for signs of wear or damage, such as cracks, corrosion, or excessive carbon buildup. If you notice any issues with a particular spark plug, it may be time to replace it.

Spray Wires with Water

Ensure that the longevity of your spark plugs is regularly spraying water on the wires connecting them to the boots. This helps prevent dirt or debris from accumulating on the surface, leading to poor electrical conductivity.

You only need a spray bottle filled with water to execute this maintenance task. With the engine turned off, locate the areas where the spark plugs attach to their respective boots. Gently spray water over these connections, making sure not to oversaturate them.

The moisture will help dislodge any particles accumulated over time and improve contact between the wires and boots. It’s important to emphasize that you should always keep the engine off while performing this task.

Use a Spark Plug Tester

The effective tool you can use to ensure that your spark plugs are in good condition is a spark plug tester. This device lets you quickly and easily check if each spark plug is firing correctly. To test for this issue, follow this simple checklist:

  • Remove the spark plug from the spark plug lead.
  • Attach the spark plug lead to a spark plug tester.
  • Attach the spark plug to the tester, establishing a connection between the lead, tester, and plug.
  • Start the engine and pay attention to any irregular timing or pauses.

Confirm Wires are Properly Routed

Maintaining your spark plugs is to confirm that the wires are correctly routed. You should refer to your vehicle’s manual and locate the correct ports for each wire to do this. If cables are improperly routed or not securely attached, it can lead to misfiring or engine damage.

To ensure the wires are in their correct place, carefully inspect each wire and follow its path from the ignition coil to the corresponding spark plug. Look for any signs of wear, such as fraying or cracks in the insulation.

Check if any of the wires have come loose or disconnected. If you notice any issues, gently reposition them according to the manual’s instructions and make sure they are securely fastened.

How to Replace Your Spark Plugs

If you are experiencing rough idling, misfires, or poor acceleration in your vehicle, replace your spark plugs. They can wear out or become fouled with carbon deposits, decreasing performance and fuel efficiency.

You can easily replace your spark plugs and restore your vehicle’s power with simple steps and essential tools.

  1. Take out all the ignition cables.
  2. Take off any parts that surround or cover the ignition cables.
  3. Find and eliminate faulty spark plugs.
  4. The proper length of spark plugs is essential. If the spark plugs are too long, they can cause harm to the engine’s pistons.
  5. Put in the new spark plugs and ensure they are correctly installed.
  6. Apply lubrication to the car’s threads before using torque for tightening.
  7. Put back all the removed parts, including the ignition coils and cables, in their respective places.

What are the Costs of Replacing Your Spark Plugs?

Cost of Parts

The cost of replacing spark plugs can vary depending on several factors. The quality and material of the spark plug greatly influence its price. Standard copper core plugs are generally cheaper, ranging from $4 to $10 per plug. Premium platinum or iridium plugs can cost anywhere from $10 to $20 each due to their enhanced durability and performance benefits. The number of cylinders in your car also affects the overall expense.

If you own an 8-cylinder car that requires $10 spark plugs, don’t be surprised when you see an $80 price tag instead. This significant price difference arises because each cylinder in your engine needs its spark plug. While a 4-cylinder car only requires four spark plugs to be replaced, an 8-cylinder car will need double that amount.

Cost of Labor

One of the factors that can significantly impact the overall cost is the labor involved. Labor costs in this area can vary substantially from one establishment to another. The variation in labor costs can stem from several factors. A mechanic with higher expertise or specialized knowledge may charge more for their services due to their reputation and experience. 

While you are fortunate enough to find a mechanic who charges as little as $50 for the job, others charge upwards of $200 or more. This discrepancy in labor costs highlights the importance of researching and contacting different mechanics for price quotes before settling on one.

How Does a Car Act When Spark Plugs Are Bad?

The most noticeable sign is a rough idling engine. As spark plugs deteriorate or become fouled, they may need help to generate enough spark to ignite the fuel-air mixture properly. This leads to an uneven firing of cylinders during idle, resulting in a smooth and consistent engine operation. Drivers may experience vibrations throughout the vehicle or even notice their steering wheel trembling at a stop.

Another symptom of faulty spark plugs is difficulty starting the car. When the ignition system fails to provide a vital enough spark, it becomes challenging for the engine to ignite and start running smoothly. The car might take longer than usual to turn over and may require multiple attempts before it finally starts up.


Recognizing the bad spark plug symptoms can help prevent further damage to your vehicle and save you from costly repairs. Regularly maintaining and inspecting your spark plugs is crucial for optimal engine performance. If you notice any of these symptoms, please have your spark plugs checked and replaced if necessary. By addressing the issue promptly, you can ensure that your vehicle continues to run smoothly and efficiently.


How long do spark plugs typically last?

Most modern spark plugs have a lifespan of about 30,000 to 100,000 miles depending on the type and quality of the plug.

Are all spark plugs the same?

Different spark plugs are available based on heat range, electrode material, and design. It’s essential to use the correct type recommended by your vehicle manufacturer for optimal performance.