What are the symptoms of a bad spark plug?

Common symptoms include engine misfires, rough idling, hard starting, reduced fuel economy, and sluggish acceleration. If left unchecked, these issues can worsen and potentially damage the catalytic converter or ignition system.

As engines evolve, spark plugs can last longer, but deterioration and fouling remain common causes of degraded performance. This article reviews the most reliable signs and how to approach diagnosis and replacement.

Common symptoms you may notice

Why these signs matter

These signs appear during regular driving and may indicate a failing spark plug or related ignition component.

• Engine misfires or runs rough, especially under load or at idle


• Difficulty starting or extended cranking


• Reduced acceleration or noticeable loss of power


• Rough idle or stalling at low speeds


• Decreased fuel economy and higher emissions


• Check Engine Light or stored misfire codes (for example P0300 and cylinder-specific P0301–P0308)

If you notice these signs, a diagnostic check is advisable. Misfires can be caused by worn plugs, faulty ignition coils, or other ignition and fuel-system faults, so testing is important to confirm the root cause.

Physical signs of spark plug wear or fouling

What to look for up close

In addition to performance symptoms, visual inspection can reveal plug wear or fouling that points to a faulty plug.

• Worn or eroded electrode gaps, which reduce spark strength


• Cracked or damaged ceramic insulation


• Deposits on the plug tip or heavy carbon buildup


• Oil-fouled or fuel-fouled plugs, which indicate engine or fuel-system issues


• Wet plugs after a failed start, suggesting improper fueling or flooding

Physical signs often accompany performance symptoms and help guide whether replacement is needed. Replacing plugs in sets and checking related components can prevent repeated issues.

Diagnosis and replacement steps

How to confirm and fix the issue

Modern engines often use long-life iridium or platinum plugs and require correct torque and gap settings. If in doubt, consult the vehicle’s service manual or a qualified technician.

• Scan for misfire codes using an OBD-II reader to identify whether a misfire is random (P0300) or cylinder-specific (P0301–P0308)


• Remove the spark plug(s) and inspect for wear, fouling, or damage; measure the electrode gap if necessary


• Check and test ignition coils and spark plug wires for damage or resistance if misfires persist


• Replace worn or fouled plugs with the correct type and heat range; ensure the plug gap matches factory specification and torque to spec


• Reinstall, clear codes, and perform a road test to confirm fixes and monitor for recurrence

After replacing, using the correct spark plug type and proper torque reduces the likelihood of recurring misfires and helps maintain engine efficiency and emissions.

Summary

Bad spark plugs typically reveal themselves through misfires, rough idle, hard starts, reduced fuel economy, and diminished power. Visual wear or fouling confirms the diagnosis, while a careful diagnostic and replacement process—with attention to correct plug type, gap, and torque—restores performance. If symptoms persist after replacement or are intermittent, seek professional inspection to rule out coil, wiring, or fuel-system issues.

How do you know when spark plugs need changing?

You know spark plugs need changing when you notice symptoms like an engine misfire, rough idling, poor acceleration, or reduced fuel economy. Other signs include a rough or noisy engine, difficulties starting the car, or the Check Engine light illuminating. Consistent vibration or hesitation while driving can also indicate a problem.
 
This video explains the signs of bad spark plugs and how to spot them: 1mHipa StoreYouTube · Aug 30, 2022
Common signs to look for:

• Engine misfires: The engine may sputter, shake, or you may feel a sudden loss of power. 
• Rough idling: The engine shakes or runs unevenly when the vehicle is stopped or idling. 
• Poor acceleration: The car feels sluggish and does not accelerate as quickly as it used to. 
• Decreased fuel economy: You may find yourself filling up the gas tank more frequently than before. 
• Difficulty starting: The engine takes longer to start or may not start at all. 
• Noisy engine: You might hear pinging, knocking, or other unusual noises, especially during acceleration. 
• Hesitation or stumbling: The vehicle jerks or hesitates when you are driving. 
• Check Engine light: A flashing or solid Check Engine light can indicate a misfire, which is a common spark plug issue. 

You can watch this video to see a demonstration of how to change spark plugs: 1mEngineering ExplainedYouTube · Apr 20, 2016
Other indications:

• A raw fuel smell from the exhaust. 
• Visible carbon buildup, a worn electrode, or other physical damage on the spark plug after removal. 
• The engine shakes so much that you feel it throughout the car. 
• A misfire code (like P0300) appearing when scanning the vehicle's computer. 

How can you tell if you have a bad spark plug?

You can tell you have a bad spark plug by observing symptoms like a rough engine idle, poor acceleration, decreased fuel economy, and engine misfires. Other indicators include difficulty starting the car, the "check engine" light coming on, and engine knocking. A visual inspection can also reveal issues like a worn or cracked electrode and buildup of oil, fuel, or carbon.
 
This video explains the symptoms of bad spark plugs and shows you what they look like: 55sHipa StoreYouTube · Aug 30, 2022
Engine performance issues

• Rough idling: The engine may shake or vibrate when it is running at a stop. 
• Engine misfires: The engine may stumble, hesitate, or stutter, especially under acceleration. 
• Poor acceleration: The car may feel sluggish and lack its usual responsiveness. 
• Decreased fuel economy: You may notice your car consuming more fuel than usual. 
• Engine knocking: In severe cases, worn plugs can lead to engine knocking, which can cause significant damage. 

Other warning signs

• Difficulty starting: The engine may crank for a longer time or struggle to start, particularly in cold weather. 
• "Check Engine" light: A flashing or illuminated "check engine" light can indicate a misfire caused by a bad spark plug. 

This video explains how to read spark plugs to diagnose engine issues: 58sMotorWeekYouTube · Apr 4, 2019
Visual inspection

• Remove and inspect: Take out the spark plug and check for physical damage. 
• Look for damage: Inspect the electrode for wear, and check the ceramic insulator for cracks. 
• Check for buildup: Look for deposits of oil, carbon, or fuel on the plug. 
• Gap: Use a spark plug gap tool to ensure the gap between the electrodes is correct, as specified in your owner's manual. 

What does it sound like when spark plugs are bad?

Higher exhaust emissions rough idling and a damaged catalytic converter. Two Poor gas mileage when spark plugs are working correctly. They burn fuel efficiently in your engine.

What happens when one spark plug goes bad?

If one spark plug is not working, your engine will misfire, leading to a rough idle, reduced power, and decreased fuel efficiency. The check engine light will likely turn on, and you may notice a loss of acceleration and a strong fuel odor in the exhaust. Driving with a faulty spark plug can also cause more serious and expensive problems over time, such as damage to the catalytic converter. 
This video explains how a failing spark plug can lead to a misfire and other engine problems: 1mEasyAutoFixYouTube · Mar 5, 2023
Immediate effects

• Engine misfire: The engine will run roughly because one cylinder isn't firing correctly. 
• Rough idle: The vehicle will vibrate noticeably, especially when idling. 
• Loss of power: You will experience sluggish acceleration and a general reduction in engine power. 
• Check engine light: The car's computer will detect the misfire and turn on the check engine light. 
• Increased fuel consumption: The engine wastes unburnt fuel, leading to lower gas mileage. 
• Fuel odor: You may smell unburnt gasoline coming from the exhaust. 

Long-term risks

• Catalytic converter damage: The excess unburnt fuel can foul and eventually destroy the catalytic converter, which is an expensive part to replace. 
• Ignition coil failure: A failing spark plug can put extra stress on the ignition coil and other components, potentially causing them to fail. 
• Other engine damage: In severe cases, extremely high combustion temperatures can damage internal engine components. 

This video shows the physical damage that can occur to an engine due to a failing spark plug: 58sRatchets And WrenchesYouTube · Sep 19, 2020
What to do

• Get it diagnosed: Take your car to an auto parts store for a free scan or to a mechanic to confirm the issue and get a specific code like "misfire on cylinder #". 
• Replace the spark plug: It's best to replace the faulty spark plug as soon as possible to prevent further damage. If one is bad, others are likely nearing the end of their lifespan as well. 
• Drive with caution: If you must drive a short distance to a repair shop, do so carefully and avoid heavy acceleration.