What are the signs of a bad spark plug?

A bad spark plug typically shows up as misfires, rough idle, hard starting, and reduced fuel economy. It can also trigger a check engine light and cause a drop in power. This article outlines the most common signs, how they manifest, and what steps to take to address the issue.

Common signs to watch for

These symptoms are the most frequently reported by drivers when a spark plug is failing, and they often appear in combination.

• Engine misfires or hesitation during acceleration


• Rough idle or engine vibration at idle


• Difficulty starting the engine, especially after long stops


• Noticeable drop in fuel efficiency and increased fuel consumption


• Engine knocking, pinging, or loss of power under load


• Check engine light with misfire-related codes (such as P0300 and cylinder-specific codes P0301–P0308)


• Rough running that feels like it’s “missing” periodically

These signs can overlap with issues in ignition coils, wires, or the fuel system, so a full diagnosis is recommended if several symptoms appear together.

Visual cues and checks

Beyond performance symptoms, a quick visual inspection and simple tests can reveal plug health or obvious wear and fouling.

• Wet, oily, or heavily carbon-fouled plugs indicating oil burning or fuel-rich running


• Worn, eroded, or widened electrode gaps that reduce spark energy


• Cracked ceramic insulation or other physical damage on the plug


• Heavy deposits, glazing, or abnormal coloration that impede spark formation


• Evidence of oil or fuel leaks reaching the plug area

Because some issues may not be visible without removing the plugs, professional inspection or a diagnostic scan is advisable if fouling or wear is suspected.

Diagnostic steps and recommended action

When signs point to a spark plug problem, a systematic approach helps confirm the fault and guide replacement decisions.

1. Inspect the ignition system components (coils, wires, boots) for cracks, corrosion, or damage.


2. Measure the spark plug gap to ensure it matches the manufacturer specification.


3. Remove and examine the plug to assess wear, fouling, and electrode condition.


4. Replace worn or contaminated plugs with the correct type and torque to spec.


5. Check related components for wear or failure to prevent a repeat issue (e.g., coils or injectors).


6. Clear codes and test drive to verify that misfires or symptoms have resolved; if problems persist, investigate other causes (fuel delivery, compression, or ignition control).

Replacement intervals vary by plug type and vehicle. Modern engines often use iridium or platinum plugs with longer service lives, commonly ranging from 60,000 to 100,000 miles in many models. Always follow the vehicle manufacturer’s recommendations for service intervals and plug type.

Choosing the right plug type

Spark plugs come in several designs, including copper, platinum, and iridium. Copper plugs are typically cheaper and may wear faster, while platinum and especially iridium plugs offer longer life and more stable performance. The correct choice depends on your engine design, heat range, and maintenance schedule provided by the manufacturer. When in doubt, consult the owner’s manual or a qualified technician to select the appropriate plug type and heat range for your vehicle.

Summary

Bad spark plugs reveal themselves through misfires, rough idle, hard starting, reduced fuel economy, and warning lights. Visual signs like fouling, worn electrodes, or cracked insulators also point to replacement. A structured diagnostic approach—checking ignition components, testing plug gaps, and validating with a drive test—helps confirm the issue and guide replacement. Using the recommended plug type and following the manufacturer’s maintenance interval can prevent these signs from arising and keep the engine running smoothly.

What happens to spark plugs when they go bad?

Faulty spark plugs result in an engine misfire due to incomplete burning fuel. An interrupted combustion process can significantly affect the engine performance. Unattended engine misfire may result in intermittent strange engine sounds, power loss, and engine knock.

How do I know if spark plugs need replacing?

You can tell spark plugs need replacing if your car has difficulty starting, a rough or shaky engine idle, sluggish acceleration, or decreased fuel efficiency. Other signs include the "check engine" light coming on, engine misfires, and unusual engine noises like knocking or pinging. 
Performance and starting issues

• Difficulty starting: The engine may crank slowly or struggle to turn over. 
• Rough idling: The engine shakes or vibrates more than usual when it's at idle. 
• Slow acceleration: The car feels sluggish and slow to accelerate when you press the gas pedal. 

This video demonstrates how a rough idle looks and sounds due to a failing spark plug: 58sEasyAutoFixYouTube · Dec 14, 2020
Efficiency and emissions

• Decreased fuel economy: You notice your gas mileage getting worse than it used to be. 
• Increased emissions: Exhaust may have more smoke than usual. 

Engine and dashboard warnings 

• Engine misfires: The engine may sputter or feel like it's misfiring, which can be accompanied by a loss of power and shaking. 
• Check Engine light: The "check engine" light can illuminate, sometimes with specific misfire codes (like P0300-P0308). 
• Engine noise: You might hear unusual sounds like knocking, rattling, or pinging, especially when accelerating. 

This video explains how a blinking check engine light can indicate a severe misfire, which might require immediate spark plug replacement: 58sEasyAutoFixYouTube · Mar 5, 2023
What to do

• Visual inspection: You can remove the spark plugs to check for visible damage like cracks, wear, or corrosion. The electrode should be clean and free of dirt. 
• Check the gap: If the gap between the electrodes is too wide, the spark plug may not function correctly. A gap tool can be used to check if it matches the specifications in your owner's manual. 
• Consult your owner's manual: Your car's manual will have information on the recommended replacement interval based on mileage and the type of spark plug you have (e.g., platinum or iridium plugs typically last longer than copper). 

What do bad spark plugs sound like?

Bad spark plugs don't always make a distinct sound, but they often cause the engine to sound and feel rough, exhibiting symptoms like a rough or irregular idle, engine misfiring, and a general stuttering or "choking" sound, especially during acceleration. Other noises that may occur include knocking or pinging, and potentially a loud, abnormal exhaust sound. 
This video shows what a rough idle caused by bad spark plugs sounds like: 56sEasyAutoFixYouTube · Dec 14, 2020
Common sounds and vibrations from bad spark plugs 

• Rough or inconsistent idle: The engine sounds shaky or jittery when it's supposed to be running smoothly at a standstill. 
• Misfiring: The engine may sound like it's momentarily cutting out or stuttering, causing a jerky feeling. 
• Knocking or pinging: This is a more serious symptom that can indicate severe engine damage is occurring. 
• "Choking" sound: Some vehicles, particularly those with louder exhausts, may make a strange choking sound when accelerating. 
• Vibrations: A rough idle can send vibrations through the car. 
• Difficulty starting: The engine may struggle to start or take longer to turn over. 

This video demonstrates how bad spark plugs can affect the engine's performance and cause knocking sounds: 59sPro WrenchingYouTube · Mar 8, 2025

How does a car act when spark plugs are bad?

A car with bad spark plugs will exhibit symptoms like rough idling, difficulty starting, poor acceleration, and decreased fuel economy. You might also notice engine misfires, a "check engine" light, or strange engine noises like knocking or rattling.
 
Common symptoms of bad spark plugs

• Rough idling: The engine may shake or feel unsteady, especially when the car is stopped. 
• Difficulty starting: The engine may crank for a longer time before starting, or require the throttle to be depressed. 
• Poor acceleration: The car may feel sluggish or "jerky" when you press the gas pedal. 
• Decreased fuel efficiency: You will likely notice you need to fill up your gas tank more often because the engine is burning fuel inefficiently. 
• Engine misfires: This can feel like an abrupt jerk or stutter and is a clear sign that a spark plug isn't firing correctly. 
• "Check Engine" light: Modern cars can detect a misfire and trigger the "check engine" light, which may be accompanied by a specific misfire code. 
• Engine knocking or rattling: A bad spark plug can cause an unusual knocking sound, particularly during acceleration. 
• Increased emissions: Incomplete combustion from faulty plugs can lead to higher levels of harmful emissions. 

This video explains the symptoms of bad spark plugs: 59shaile car care YouTube · Apr 23, 2023
What to do
If you experience these symptoms, it's important to have your spark plugs checked, especially if they are past the manufacturer's recommended replacement interval. Continued driving with bad spark plugs can lead to further engine damage. 
You can watch this video to learn how to identify bad spark plugs: 58sEasyAutoFixYouTube · Mar 5, 2023