How do I know if my spark plugs need replacing?

If your engine is hard to start, runs rough, misfires, or your fuel economy has dropped, your spark plugs may be wearing out and due for replacement. Check your vehicle’s manual for the recommended replacement interval and inspect the plugs if you notice any symptoms.

Spark plugs ignite the air-fuel mixture in each cylinder, and over time their electrodes wear, gaps widen, and deposits build up. This can degrade performance, increase emissions, and shorten engine life. The best way to know for sure is to pay attention to symptoms, perform a visual inspection, and use basic diagnostics if needed. The guidance below walks you through signs, how to inspect plugs, typical replacement intervals by plug type, and practical steps for replacement or professional help.

Common symptoms that spark plugs may be worn

These signs commonly point to worn or fouled plugs. If several apply, it’s wise to check or replace the plugs to restore performance.

• Difficulties starting the engine or extended cranking before it fires


• Rough idle or engine misfires, especially at light load


• Poor acceleration or reduced power and responsiveness


• Lower fuel economy or higher fuel consumption


• Engine knocking, pinging, or hesitation under load


• Check Engine Light with misfire codes (P0301–P0306, P0300, etc.)


• In cold weather, sluggish starts or misfiring becomes more noticeable

When you notice a combination of these symptoms, it’s reasonable to inspect the plugs or have a technician check the ignition and fuel system to confirm wear or fouling.

How to inspect spark plugs

Before you dive in, know what to look for and how to assess wear without jumping to conclusions. This helps determine whether replacement is warranted.

• Remove each plug in sequence and inspect the electrode tip and porcelain for wear or damage


• Check the gap with the correct feeler gauge and compare it to the manufacturer’s specification


• Look for signs of fouling: oil deposits, fuel deposits, or heavy carbon buildup


• Inspect for damaged or cracked ceramic insulator or a burnt electrode


• Notice if several plugs show similar wear, suggesting an ignition or engine issue that affects all cylinders

After inspecting, you’ll have a clearer idea whether individual plugs can be cleaned or should be replaced and whether the entire set should be changed at the same time for consistency.

Replacement intervals and best practices

Intervals vary by plug material, engine design, and driving habits. In most modern cars, the manufacturer’s recommended interval is the starting point, but real-world conditions can shorten or extend life.

• Copper plugs: commonly replaced around 20,000–40,000 miles for many vehicles, though some cars may go longer depending on engine design and maintenance history


• Platinum plugs: typically rated for about 60,000–100,000 miles, offering longer life than copper in many applications


• Iridium or advanced long-life plugs: often 100,000 miles or more; some OEMs list 120,000 miles


• Direct-injection or turbocharged engines: carbon buildup can shorten plug life; expect earlier checks and possible cleaning or replacement


• Driving style and conditions (short trips, heavy loads, frequent cold starts) can shorten plug life regardless of type

Always prioritize the manufacturer’s specification. If you’re unsure, consult your owner's manual or a professional to set an appropriate replacement interval for your specific engine and plug type.

What to do if you’re not sure

If you’re uncertain about the condition of your plugs after inspection or diagnostics, consider these options. You can perform some checks yourself or seek professional help for a definitive assessment.

Consult a professional and use diagnostics

Mechanics can confirm misfires or ignition problems with diagnostic scans and, if needed, a compression test to rule out more serious engine issues. Codes indicating misfires can guide you to the affected cylinder and help determine whether the plugs are at fault or if other components (coils, wires, or injector) are involved.

DIY replacement tips

If you choose to replace plugs yourself, follow these practical tips:

• Ensure the engine is cool before removing spark plugs to avoid damaging threads


• Disconnect the battery and label/remap any ignition coil or boot connections to avoid miswiring


• Use the correct type and heat range of plug specified by the manufacturer; many are pre-gapped and do not require gap adjustment


• Lightly oil or anti-seize compounds are typically not needed on modern aluminum heads and can cause over-torque; follow manufacturer guidance


• Torque plugs to the recommended specification using a torque wrench; overtightening can damage threads


• Apply dielectric grease to the inside of the boots to ensure a good seal and easier removal next time


• Replace plugs in a complete set when one is worn or fouled to maintain uniform performance across cylinders


• Start the engine after installation and check for smooth operation; address any misfires or codes promptly

Replacing plugs in a timely, properly executed manner can restore performance, improve fuel efficiency, and reduce emissions. If you’re unsure, it’s wise to seek professional help to avoid common installation errors.

Summary

Spark plug wear shows up as hard starts, rough idle, misfires, reduced power, and lower fuel economy. Inspect plugs visually for wear and deposits, check the gap against manufacturer specifications, and consider the service interval dictated by plug material and engine design. Copper plugs often require earlier replacement than platinum or iridium variants. Direct-injection engines may experience quicker buildup, prompting earlier checks. When in doubt, rely on the owner's manual, diagnostic codes, and, if needed, a qualified technician to confirm whether replacement is necessary. Regular, systematic checks help keep your engine running smoothly and efficiently.

Can you check spark plugs without replacing them?

You can check if spark plugs are firing without removing them by using an inline spark tester or a diagnostic scanner to identify misfires. To test for spark directly, connect a spark plug tester between the ignition coil and the plug, or hold a screwdriver near the wire (away from yourself) and a ground while cranking the engine to look for a visible spark. While these methods confirm a spark, they don't allow for visual inspection of the plug's condition, which is a key benefit of removing it. 
This video demonstrates how to check for spark without removing the spark plugs: 46sR&R RestorationsYouTube · Apr 26, 2022
Use a spark plug tester 

1. Attach the tester: Place an inline spark plug tester between the spark plug wire and the spark plug.
2. Turn the ignition: Turn the ignition on, or crank the engine if it won't start.
3. Look for a spark: Watch the tester's transparent side for a visible spark or glow. A spark indicates the ignition system is producing a spark, but a lack of spark could mean a bad plug or another issue. 

Use a diagnostic scanner 

1. Connect the scanner: Plug an engine diagnostic scanner into your car's OBD-II port. 
2. Scan for codes: Run a scan to check for misfire codes. The scanner can often identify which cylinder is misfiring. 

This video shows how to diagnose a misfire with a spark plug wire: 53s1A Auto: Repair Tips & Secrets Only Mechanics KnowYouTube · Nov 29, 2022
Use a screwdriver (use with caution) 

1. Locate the ignition wire: Find the wire going to the spark plug you want to test.
2. Create a gap: With the ignition on, carefully hold the metal shank of a screwdriver near the spark plug boot or coil and maintain a gap of about 1/81 / 81/8 of an inch to a good engine ground.
3. Check for a spark: Watch for a spark to jump from the screwdriver to the ground. Do not hold the screwdriver with your hand while performing this test. 

This video explains how to check for spark with a screwdriver: 59sProblem Solver GarageYouTube · Feb 20, 2023
Listen for symptoms

• Listen for engine problems such as misfires, rough idling, or a shaking engine, which could indicate a spark plug issue.
• Note if you experience decreased fuel mileage, stumbling, or stalling. 

What are the symptoms of spark plugs needing replaced?

Key signs include rough idling, sluggish acceleration, engine misfires, difficulty starting, and a noticeable drop in gas mileage. Spark plug lifespan varies significantly by type — copper plugs may need replacement every 25,000-30,000 miles, while platinum or iridium plugs often last 100,000 miles or more.

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

How often do spark plugs need replacing?

Spark plugs should be changed based on your vehicle's owner's manual, but a general guideline is every 30,000 to 100,000 miles, depending on the type of plug. The most accurate answer is to check your owner's manual, as plug types vary from copper (shorter life) to iridium (longer life) and driving conditions also impact wear. 
General recommendations

• Owner's manual: This is the most reliable source for your specific vehicle. 
• Copper plugs: Typically need replacement every 20,000–30,000 miles. 
• Platinum or Iridium plugs: These are standard in many modern vehicles and can last much longer, sometimes up to 100,000 miles or more. 

Signs your spark plugs may need changing 

• Rough idle: The engine feels shaky, bouncy, or unstable when stopped. 
• Misfiring: A hesitation or jerking sensation when accelerating. 
• Reduced fuel efficiency: You notice a sudden drop in your miles per gallon (MPG). 
• Difficulty starting: The engine cranks for a long time before starting, or takes longer than usual to start. 
• Check Engine light: A steady or flashing "Check Engine" light can indicate a misfire caused by failing spark plugs. 

Factors that can shorten plug life

• Demanding driving habits: Frequent short trips, aggressive acceleration, and driving in extreme temperatures can reduce plug lifespan. 
• Vehicle modifications: Aftermarket engine tuning can require more frequent spark plug changes.