What are signs of bad spark plugs?

Common signs of bad spark plugs include rough idle, engine misfires, hard starting, reduced power and fuel economy, and a check engine light.

Spark plugs ignite the air-fuel mixture in each cylinder. When they wear, foul, or the gap widens, the spark energy drops and combustion becomes inconsistent. This can lead to a range of performance and reliability issues that vary by vehicle and plug type, but the core problem is weaker or mistimed ignition affecting one or more cylinders.

Common symptoms to watch for

The following indicators are typically reported by drivers and mechanics when spark plugs are nearing the end of their life or have become fouled.

• Rough idle or noticeable vibration when the engine is idling


• Difficulty starting or repeated hard starts


• Engine misfires, especially under load, during acceleration, or at idle


• Reduced power or sluggish acceleration


• Lower fuel economy and higher fuel consumption


• Check Engine Light illuminating, with diagnostic codes such as P0300 (random misfire) or P0301–P0304 (misfires in specific cylinders)


• Engine surging or bucking under light or steady throttle


• Stalling or occasional rough running in some driving conditions


• Longer cranking times in cold weather


• Increased emissions or emissions-test failure due to incomplete combustion

These signs suggest the ignition system is not reliably delivering a spark at the right time, and you should have the plugs inspected and tested.

Visual signs and diagnostic clues

Beyond driving symptoms, there are tangible signs you can look for during a visual inspection or a diagnostic check.

• Deposits on plug electrodes, such as carbon buildup (sooty black) indicating rich running or incomplete combustion


• Oil fouling on the plug tip or threads, which can point to oil entering the combustion chamber or worn valve seals/PCV system


• Worn or eroded electrode gaps, which reduce spark energy and shorten plug life


• Damaged or cracked ceramic insulator, which can cause misbehavior or misfires


• Discolored or damaged plug sheath, or a visibly oily or fouled plug in multiple cylinders


• Ignition system wear signs (such as worn boots or damaged coil-on-plug connectors) that can accompany plug wear

Visual signs alone don’t confirm failure, but they strongly suggest that a plug should be tested or replaced as part of preventive maintenance.

How to determine whether to replace spark plugs

To decide if replacement is needed, follow these practical steps or consult a technician.

• Check the vehicle’s service manual for the recommended replacement interval and plug type


• Perform a spark plug inspection: remove a plug and assess the electrode condition, gap, and deposits


• Measure plug gap with a feeler gauge and compare to the manufacturer’s specification


• Consider replacement if the gap is widened beyond spec, deposits are heavy, or the plug shows wear or damage


• If misfires persist after replacing plugs, inspect related components (wires/coil packs, ignition coils, fuel injectors, compression) or run a diagnostic scan


• Have a professional service if you’re unsure or if the vehicle has high-mileage or complex ignition systems

Regular inspection and timely replacement of spark plugs can restore performance, improve fuel economy, and protect the exhaust system from damage caused by misfires.

Summary

Bad spark plugs typically manifest as rough idling, misfires, hard starting, power loss, and lower fuel efficiency, often accompanied by a check engine light and specific codes. Visual signs such as carbon or oil fouling, worn electrodes, or a damaged insulator further point to the need for replacement. Following the vehicle’s maintenance schedule and performing periodic inspections helps maintain reliable ignition, optimal performance, and cleaner emissions.

How do I know if my spark plugs need changing?

You can tell your spark plugs need changing by a combination of symptoms, including rough idling, difficulty starting, poor acceleration, decreased fuel economy, and engine misfires. The "check engine" light can also turn on, especially if it is flashing. A physical inspection of the old plugs can also reveal damage like wear, cracks, or fouling. 
Performance symptoms

• Rough or jittery engine: If the engine shakes or vibrates more than usual, especially at idle, it could be a sign of a misfiring spark plug. 
• Engine misfires: You might hear or feel a "sputtering" or a sudden loss of power. This happens when a spark plug fails to ignite the air-fuel mixture. 
• Poor acceleration: A sluggish feeling when you press the gas pedal can indicate weak spark from worn-out plugs. 
• Difficulty starting: Worn spark plugs may struggle to create a strong enough spark, leading to a longer crank time or trouble starting the car at all. 
• Decreased fuel economy: If you notice you're visiting the gas station more often, faulty spark plugs could be the cause because they lead to incomplete combustion and wasted fuel. 

You can watch this video to learn about the symptoms of failing spark plugs: 59sTomorrow's TechnicianYouTube · Mar 16, 2021
Other indicators

• "Check Engine" light: This is a common warning sign. A steady light can indicate a misfire, while a flashing light can signal a more serious problem, like a misfire that could damage your catalytic converter, requiring immediate attention. 
• Engine noise: Unusual sounds like pinging or knocking can be a sign of ignition problems caused by bad spark plugs. 

How to confirm

• Visual inspection: The best way to confirm is to remove the spark plugs and check for visible signs of wear, such as cracks, corrosion, or a rounded electrode. The tip should be clean, not fouled with oil or carbon buildup.
• Check the gap: Use a spark plug gap tool to ensure the gap between the electrodes is within the manufacturer's specifications, which are listed in your owner's manual. 

This video demonstrates how to inspect and test spark plugs: 1mEngineering ExplainedYouTube · Apr 20, 2016

Can I drive with bad spark plugs?

You can drive short distances with bad spark plugs, but it's not recommended for an extended period as it can cause engine damage and lead to more expensive repairs. Bad spark plugs can cause a rough idle, reduced acceleration, and poor fuel economy, but the most significant risk is that unburnt fuel can damage the catalytic converter. If the check engine light is flashing, you should stop driving immediately. 
Short-term effects

• Rough driving: You may notice a jerky or sluggish feeling, particularly during acceleration. 
• Poor fuel efficiency: Your car will use more gas because the engine isn't burning fuel efficiently. 
• Hard starting: The engine may take longer to start, as misfires make it harder for the engine to get going. 

Risks of driving with bad spark plugs

• Engine misfires: These can damage pistons and valves over time. 
• Catalytic converter damage: Unburnt fuel can pass into the catalytic converter, leading to overheating and expensive damage. 
• Long-term engine damage: Continued driving with bad spark plugs can cause more serious and costly issues with the engine's internal components. 

When to stop driving immediately

• Flashing check engine light: A flashing check engine light, which can be triggered by a misfire, is a sign of a serious problem and you should not continue driving. 

What to do

• If you notice symptoms, it's best to replace the spark plugs as soon as possible to prevent further damage. 
• If you must drive, do so gently at low speeds and avoid heavy acceleration. 
• If the check engine light is flashing, pull over safely and have the vehicle towed to a repair shop. 

What are the symptoms of a bad spark plug?

Symptoms of using the wrong spark plug include engine misfires, rough idling, poor acceleration, decreased fuel economy, and difficulty starting the engine. A flashing or steady "Check Engine" light can also indicate a problem, as the engine's computer detects suboptimal combustion. 
Performance issues

• Engine misfires: A sudden loss of power or a stuttering, jerking sensation, especially when accelerating. 
• Rough idling: The engine shakes or vibrates when the car is stopped, instead of running smoothly. 
• Poor acceleration: The car feels sluggish and doesn't respond quickly when you press the gas pedal. 
• Engine knocking or pinging: Unusual sounds from the engine can indicate improper combustion. 

Fuel and starting problems

• Decreased fuel economy: Your vehicle uses more gas than usual due to inefficient combustion. 
• Difficulty starting: The engine cranks longer than normal or struggles to start, particularly in cold weather. 

Diagnostic indicators

• "Check Engine" light: The light may come on, sometimes flashing, which signals that the engine control module has detected a problem with combustion. 

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