How to tell if your spark plug is bad

You can usually tell a spark plug is bad if you notice misfires, rough idling, poor acceleration, worse fuel economy, or a check-engine light (often with codes like P0300–P0308). Confirm by scanning for codes, inspecting the plug’s tip for fouling or wear, checking the gap against spec, and swapping the plug to see if the misfire follows that cylinder.

What a failing spark plug looks and feels like on the road

Before opening the toolbox, pay attention to what the car is telling you. Bad plugs weaken or disrupt the spark, which shows up in predictable ways during daily driving.

• Rough idle, shudder, or intermittent shaking—especially on cold starts.
• Hesitation or stumbling under acceleration or heavy load; reduced power.
• Poor fuel economy and a stronger fuel smell from the exhaust.
• Hard starts or longer cranking, particularly in damp or cold weather.
• Check-engine light: P0300 (random misfire) or cylinder-specific P0301–P0308.
• Flashing check-engine light under load indicates an active misfire that can damage the catalytic converter—reduce load and diagnose immediately.

These symptoms don’t prove the plug is bad, but they strongly narrow the suspect list, which also includes ignition coils, wires/boots, injectors, vacuum leaks, and compression issues.

Quick checks before you pull a plug

A few non-invasive checks can confirm a misfire and often point you to the likely cylinder before you remove anything.

• Scan for OBD-II codes and freeze-frame data; note P030X cylinder numbers and conditions (RPM, load, temp).
• Use live data misfire counters (if available) to see which cylinder is racking up misfires.
• Listen for ticking or snapping sounds in the dark around coils/wires—arcing indicates insulation breakdown.
• Inspect coil boots/wires for oil in the plug wells, cracks, or carbon tracks on the ceramic.
• Watch fuel trims: a single-cylinder misfire with normal trims often points to spark; very rich trims or fuel smell may indicate unburned fuel from misfires.

If data points to a single cylinder, you can focus your physical inspection on that plug and coil first, saving time.

Hands-on inspection: confirming a bad spark plug

Tools you’ll want

Having the right tools makes inspection accurate and prevents damage to aluminum cylinder heads.

• Spark plug socket (correct size) with extension and ratchet; magnetic or rubber insert helps extraction.
• Torque wrench capable of low values suitable for spark plugs.
• Feeler or wire-type gap gauge matching your vehicle’s spec.
• OBD-II scanner (for codes and misfire counters).
• Compressed air to blow out debris from plug wells.
• Dielectric grease (for inside coil boots), new coil boots if cracked.
• Optional: multimeter for continuity/resistance checks; inspection light.

These tools let you remove, read, measure, and properly reinstall plugs without overtightening or cross-threading.

Safe removal and basic procedure

Take care during removal; aluminum heads and fine-wire plugs are easy to damage if rushed.

1. Work on a cool engine. Disconnect the negative battery cable if your layout makes accidental cranking possible.
2. Label and remove coil connectors (or plug wires). Pull coils straight up; don’t pry on the porcelain.
3. Blow out dirt from plug wells so debris doesn’t fall into the cylinder.
4. Use the spark plug socket to loosen and remove the plug carefully. If it fights, stop—apply penetrating oil, allow to soak, and try again.
5. Inspect the plug; keep track of which cylinder it came from.

A methodical approach prevents contamination, avoids damaging threads, and preserves evidence you’ll read on the plug tip.

Reading the plug: what good and bad look like

The ceramic insulator nose and electrodes tell a detailed story about combustion, heat, and oil/fuel conditions.

• Normal: Light tan/gray deposits, sharp electrode edges, dry and clean; gap near spec.
• Carbon fouling (rich/short trips/weak spark): Dry, sooty black deposits; may cause cold-start misfires.
• Oil fouling (worn rings/valve seals): Wet, oily black plug; exhaust may smoke blue; misfire often at idle.
• Ash deposits (oil/burned additives): Light tan/whitish crusty deposits; can cause misfire at high load.
• Fuel fouling (no spark or very rich): Wet with gasoline smell; plug won’t fire until dried or replaced.
• Overheating/lean: Chalky white insulator, blistering, melted or receded electrode; can precede pre-ignition.
• Detonation/pre-ignition damage: Cracked insulator, broken or hammered electrode—replace immediately and find the cause.
• Cracked porcelain or carbon tracking lines: Allows spark to leak to the shell—misfire likely under load.
• Excessive gap/rounded electrode: Wear increases gap; weak coil struggles to jump it, causing misfires.

If your plug shows heavy fouling, physical damage, or severe wear, it’s effectively “bad” and should be replaced—after you address the underlying cause.

Measurements and tests to confirm

Simple checks can separate a worn plug from other ignition problems and prevent repeat failures.

• Gap check: Measure with a wire/feeler gauge and compare to the factory spec (often around 0.7–1.1 mm / 0.028–0.044 in, but verify your exact model). If a fine-wire iridium/platinum plug is out of spec, replace—don’t re-gap aggressively.
• Swap test: Move the suspect plug to another cylinder and clear codes; if the misfire follows, the plug is the culprit.
• Coil swap A/B test: If the misfire stays with the cylinder after swapping the plug, swap the coil; if it follows the coil, the coil is bad.
• Continuity/resistance: Some resistor plugs show a few thousand ohms through the center electrode, but designs vary and this isn’t definitive; infinite/open or physically loose terminals are a red flag.
• Boot and seal check: Replace cracked boots; light dielectric grease inside the boot helps sealing and future removal (never on the electrodes).

The swap test is the most reliable DIY method to isolate a bad plug versus a coil or injector—quick, cheap, and conclusive.

Spark plug vs. coil or injector: telling them apart

Misfires don’t always mean a bad plug. A few patterns help narrow down the root cause.

• Under-load misfire that tracks with plug swaps: Typically plug or coil; if it follows the coil, the coil is failing under stress.
• Idle-only misfire with oily plug: More likely oil fouling or low compression than a simple worn plug.
• Dead miss with fuel-wet plug but good spark: Suspect injector stuck open or severe mixture problem.
• Lean codes and multi-cylinder misfires: Look for vacuum leaks or low fuel pressure before blaming plugs.
• Cold-start only misfire that clears warm: Mild carbon fouling or marginal coil/boot insulation.

Combining scan data, plug reading, and the swap test quickly identifies whether you’re dealing with spark, fuel, or mechanical issues.

When to replace and best practices

Even if your plugs aren’t “bad,” age and mileage degrade performance. Follow the factory maintenance schedule and proper installation steps to avoid problems.

• Intervals (typical): Copper 20–30k miles; platinum ~60k; iridium 80–120k—always confirm your vehicle’s OEM interval.
• Use the exact OEM-specified plug type, heat range, and reach; mixing types can cause drivability or engine damage.
• Replace all plugs as a set; consider new coil boots on high-mileage coil-on-plug setups.
• Gap only if the plug type and maker allow it; many fine-wire iridium plugs are pre-gapped and easily damaged.
• No anti-seize on modern nickel-plated plugs unless the manufacturer explicitly says so; it alters torque and risks over-tightening.
• Torque to spec; if specs are unavailable, follow the plug maker’s angle method for new gasketed plugs (snug, then typically 1/2–2/3 turn). Avoid overtightening in aluminum heads.
• Apply a small amount of dielectric grease inside the boot, not on the electrode or threads.

Correct parts and installation prevent repeat misfires, protect threads, and ensure a reliable spark under load.

FAQs

Can a bad spark plug damage the catalytic converter?

Yes. Persistent misfires send unburned fuel into the exhaust, which can overheat and melt the catalytic converter. A flashing check-engine light means stop heavy acceleration and fix the misfire promptly.

Can I clean and reuse a fouled spark plug?

You can remove light carbon with a soft brush and brake cleaner, but abrasive blasting can damage the insulator glaze. If a plug is oil- or fuel-fouled, cracked, or significantly worn, replacement is best—and fix the root cause to prevent repeat fouling.

How often should I check my spark plugs?

Inspect at the factory interval or if misfire symptoms appear. For vehicles with extended-life iridium plugs, a mid-interval check (e.g., around 60k–80k miles) can catch coil boot deterioration or early fouling, especially on engines that see short trips.

Summary

To tell if a spark plug is bad, watch for drivability symptoms and scan for misfire codes, then confirm by inspecting the plug for fouling, cracks, or wear and checking the gap. Use a swap test to see if the misfire follows the plug. Replace with OEM-specified plugs torqued to spec, avoid anti-seize unless directed, and address underlying fuel, oil, or ignition issues to prevent recurrence.

What can be mistaken for bad spark plugs?

Other common culprits to rule out are issues with the ignition coils, fuel system, or engine sensors. However, these signs indicate it is time to investigate, and spark plugs are usually the first to check and diagnose.

How do you check if a spark plug is bad?

To check for bad spark plugs, listen for an inconsistent engine sound or rough idle, feel for a lack of acceleration, and look for the “check engine” light. You can also visually inspect the removed spark plugs for physical damage, carbon or oil buildup, or a cracked insulator. To confirm, use a spark plug tester to check for a strong blue spark or use a multimeter to test the plug’s resistance. 
Symptoms of Bad Spark Plugs

• Rough Idling: The engine may shake or vibrate when stopped. 
• Engine Misfires: The engine might hesitate, jerk, or stutter when driving. 
• Difficulty Starting: Your car may struggle to start because the spark plugs aren’t igniting the fuel properly. 
• Poor Fuel Economy: Worn-out or faulty spark plugs can lead to increased fuel consumption. 
• Reduced Acceleration: Your vehicle may feel sluggish or slow to respond when you press the gas pedal. 
• Check Engine Light: A faulty spark plug can trigger your vehicle’s check engine light, indicating an ignition problem. 
• Black Exhaust Smoke: Excessive smoke from the exhaust can indicate an issue with the ignition or fuel system, potentially due to a bad spark plug. 

How to Test Spark Plugs

1. Visual Inspection:
   • Disconnect: the spark plug wire from the top of the plug. 
   • Remove: the spark plug from the engine. 
   • Examine: the plug for signs of:
     • Damage: Look for cracks, chips, or wear on the electrode. 
     • Fouling: Check for excessive carbon buildup or oil on the plug. 
     • Corrosion: Inspect for any signs of rust or corrosion. 
     • Cracked Insulator: Look for cracks in the porcelain part of the plug. 
     • Incorrect Gap: Measure the gap between the center and ground electrodes with a spark plug gap tool and compare it to your owner’s manual specifications. 
2. Spark Plug Tester:
   • Attach the tester to the removed spark plug and ground it against the engine block. 
   • Crank the engine to look for a spark. 
   • You should see a strong, blue spark. A yellow spark or no spark at all indicates a bad plug. 
3. Multimeter Test:
   • Set a multimeter to the ohms (Ω) setting. 
   • Touch one probe to the metal terminal at the tip of the plug and the other to the metal part where the wire connects. 
   • A normal reading for a plug with a resistor is 5-15 ohms; a non-resistor plug should read 1-5 ohms. 
   • No reading or a reading on the wrong parts of the plug indicates a problem. 

What to Do If a Plug is Bad

• Clean: the plug with a wire brush or brake cleaner if it only has minor buildup. 
• Adjust: the spark plug gap if it is too wide. 
• Replace: the spark plug if it shows signs of damage, is fouled, or has an incorrect gap. 

What does a bad spark plug sound like when accelerating?

A bad spark plug on acceleration can sound like sputtering, popping, or knocking, and it will often lead to poor performance, such as slow acceleration and a loss of power. You may also notice a general rough feeling or vibration from the engine, a delayed response to the gas pedal, and reduced gas mileage. 
Common Sounds and Symptoms

• Sputtering: If the spark plug isn’t producing a strong enough spark, the engine may sputter or hesitate when you accelerate. 
• Pinging or Knocking: Premature or uneven ignition of the fuel-air mixture, known as detonation, can cause a metallic pinging or knocking sound, often under load or during acceleration. 
• Popping/Backfiring: A backfire can occur when a spark plug fires at the wrong time, causing a loud pop or bang from the engine. 
• Slow Acceleration: The engine may not react as quickly as usual when you press the gas pedal. 
• Loss of Power: You might feel a sudden drop in the vehicle’s power. 
• Roughness: The engine might feel jerky or vibrate more than usual. 

Why It Happens

• Misfires: The spark plug fails to ignite the fuel-air mixture, leading to incomplete combustion. 
• Incorrect Spark Timing: The spark plug fires too early or too late in the combustion cycle. 
• Detonation (Spark Knock): The fuel-air mixture explodes unevenly, creating a pinging noise. 
• Engine Misfiring: Caused by issues like a dirty or damaged spark plug, leading to a loss of power and poor performance. 

What to Do 

• Schedule a Service Appointment: It is recommended to have a qualified technician diagnose the problem, as other issues like dirty fuel injectors or a bad fuel pump can cause similar symptoms.

What are the symptoms of bad spark plugs?

Symptoms of a failing or damaged spark plug include a check engine light, rough engine idling and vibration, engine misfires, difficulty starting the vehicle, poor acceleration, reduced fuel economy, and the potential for the engine to knock or make unusual sounds. Visual signs of damage can include carbon buildup, oil fouling, or cracks on the plug.
 
Performance Issues

• Rough Idling: A healthy engine idles smoothly, but a damaged spark plug can cause the engine to run rough and vibrate at idle. 
• Engine Misfires: You might feel a sputtering, jerking, or hesitation in the engine, as the spark plug fails to ignite the fuel-air mixture, causing a loss of power in a cylinder. 
• Poor Acceleration: The engine may feel sluggish, struggle to gain speed, and respond slowly when you press the accelerator. 
• Difficulty Starting: Worn-out spark plugs can fail to create the necessary spark to start the engine, leading to extended cranking times, especially in cold weather. 

Fuel & Efficiency Issues

• Poor Fuel Economy: Worn spark plugs can lead to inefficient fuel combustion, causing you to visit the gas station more often. 
• Increased Emissions: Inefficient burning of fuel due to bad spark plugs contributes to higher exhaust emissions. 

Warning Signs

• Illuminated Check Engine Light: Opens in new tabModern vehicles are equipped to detect issues with spark plugs, which can trigger the check engine light to illuminate on the dashboard. 
• Engine Knocking or Unusual Sounds: Opens in new tabIn some cases, bad spark plugs can cause the engine to make unusual noises like pinging, rattling, or knocking sounds. 

Visual Damage 

• Carbon Buildup: Deposits of carbon, oil, or fuel can accumulate on the spark plug, fouling it and preventing proper function. 
• Cracks or Chips: Cracks or chips in the spark plug’s insulator can allow the electrical current to escape, causing misfires.