Not Sure When to Change Spark Plugs? Look for 6 Signs (2023)

Many drivers assume that when their car has trouble starting, there’s a problem with the battery. While this certainly could be the case, it’s not the only possibility. Old or faulty spark plugs could be the culprit! After all, these small spark plugs have a big job to do. Learn why it’s important to keep this part of your car in great working order and when to change spark plugs by watching for these bad spark plug symptoms.

Changing spark plugs could be a good idea when...

1. Your check engine light comes on.

Worn/failed spark plugs, coil packs, or spark plug wires can trigger your check engine light to come on. In today's cars, say Firestone Complete Auto Care technicians, if a plug starts to fail then the most obvious event should be the check engine light coming on or even flashing.

This can cause your engine to misfire and your check engine light to pop on. A flashing engine light could mean that potentially catastrophic misfires are taking place, say our technicians.

(Video) 5 Signs It's Time To Replace Your Spark Plugs

In general, it's best to replace spark plugs as part of preventative maintenance based on manufacturer's specifications. This can help save you from costly repairs, as driving with misfiring spark plugs could put undue stress on your car's catalytic converter (the engine's exhaust cleaner).

Symptoms of misfiring spark plugs include rough idling, uneven power when accelerating, and an increase in exhaust emissions.

Note that a check engine light could be potentially caused by a number of things, not just a spark plug problem. That's one reason your local Firestone Complete Auto Care offers Code Scans, which reveal any codes that indicate a misfire due to a spark plug failure. Since our facilities provide complete auto care, our technicians also have the proper tools and expertise in engine control systems, sensors, and on-board diagnostics to perform an advanced diagnostic and provide an accurate repair estimate.


2. Your car has trouble starting.

Your spark plugs have one of the most important jobs in your vehicle, which is to provide the spark that powers the engine! Old, worn out spark plugs have a harder time creating the spark that actually powers your engine. If your car stalls when you are trying to turn it on, there could be a problem with spark plugs or damaged spark plug wires. The battery is most likely the culprit if your car has no power and won't start at all.

3. You’re filling up the gas tank more often.

If your spark plugs are on their last leg, they’ll take it out on your gas mileage. Worn out spark plugs can increase your vehicle’s fuel consumption because they won’t effectively burn the fuel that goes into the engine, meaning you’ll be shelling out more money on fill-ups.

"A misfiring spark plug can reduce fuel efficiency by as much as 30%," reports the National Institute for Automotive Service Excellence.

(Video) How To Tell When You Need New Spark Plugs

In general, keeping your car tuned up can work wonders for your fuel efficiency.

"Fixing a car that is noticeably out of tune or has failed an emissions test can improve its gas mileage by an average of 4%," reports

4. The engine idles roughly (and you can hear it!).

Typically (hopefully!) your engine probably purrs like a kitten, but turn down your music and take a listen. If you notice that the engine is making rattling, pinging, or knock-like noises, your spark plugs might be to blame.

(Video) 11 Symptoms of a Bad Spark Plug Wires in Cars (signs of bad spark plug wires)

5. Your car won’t accelerate quickly.

You’re probably pretty familiar with how your vehicle handles and drives. If you notice that your car isn’t as responsive as normal, particularly when you’re trying to accelerate, this could be traced back to worn spark plugs. It might be time to have them replaced – no one likes driving a sluggish, fuel-guzzling car.

6. Per the manufacturer, it's time!

In any car care matter, it's always best to follow the Manufacturer Suggested Maintenance Schedule. Review your vehicle's suggested maintenance schedule to see when to replace your spark plugs.

Luckily, spark plugs don’t wear out very quickly. You can typically get 80,000 miles on them before they need replacing. But if you notice any of these symptoms, it’s time to get your spark plugs checked out with an engine tune up.

(Video) How to "Read" your Spark Plugs | Goss' Garage

Continuing to drive on worn out or damaged spark plugs can ultimately cause engine damage, so don’t put it off. Make an appointment at your local Firestone Complete Auto Care so our expert technicians can see exactly what’s going on!


How do I know if my spark plugs need replacing visually? ›

Blisters on the insulator tip, melted electrodes, or white deposits are signs of a burned spark plug that is running too hot. Causes can include the engine overheating, incorrect spark plug heat range, a loose spark plug, incorrect ignition timing or too lean of an air/fuel mixture. The spark plug should be replaced.

Do I have to replace all 6 spark plugs? ›

Q: Should You Replace All Spark Plugs At Once? A: Yes, as a general rule, it's better to replace all plugs at the same time to ensure consistent levels of performance.

What happens if I only change 3 out of 6 spark plugs? ›

CHanging just three of them? The engine will run better than leaving all the old spark plugs in but not as well as with 6 new plugs.

Can you tell a difference after changing spark plugs? ›

In most cases, you'll see a noticeable difference in performance after replacing your spark plugs and wires. The engine will run smoother and faster, and the car will have better acceleration. You may also notice an improvement in fuel economy.

What do really bad spark plugs look like? ›

Black, chunky carbon buildup spanning the gap between the sparking end of the plug and the hook-shaped metal piece overhanging the end. Dark carbon residue built up on the ends but not spanning the gap. Oily residue on the end (most likely to happen with two-stroke engines that use a gas/oil mixture).

How long can you drive with bad spark plugs? ›

You can typically get 80,000 miles on them before they need replacing. But if you notice any of these symptoms, it's time to get your spark plugs checked out with an engine tune up. Continuing to drive on worn out or damaged spark plugs can ultimately cause engine damage, so don't put it off.

What happens if you drive with bad spark plugs? ›

Driving With Faulty Spark Plugs

Malfunctioning spark plugs cause engine problems including, misfires, hard starts, reduced gas mileage, rough idling, and lack of acceleration. Driving with a faulty spark plug will be difficult because the engine might fail to function.

Is it OK to change only 2 spark plugs? ›

Misfiring due to excessive heat, quenching, increased fuel consumption and even engine failure can occur from the use of incorrect or mismatched spark plugs, so it's best to replace all of your spark plugs at the same time, and with the correct type of spark plug, to ensure smooth and consistent operation.

How much does it cost to replace all 6 spark plugs? ›

You can pay between $16 and $100 for a set of spark plugs, depending on the type of plugs you need and the number of cylinders your engine has. And when figuring out the price, it helps to remember that some specialty engines use two spark plugs per cylinder.

What happens if you don't replace all spark plugs at once? ›

If you let your spark plugs go for too long, you could do some severe damage to your engine. The deposits that build upon the plugs can eventually cause them to misfire. When this happens, the unburned fuel in the cylinders can start to eat away at the cylinder walls.

How long does it take to replace 6 spark plugs? ›

Replacing spark plugs will take roughly an hour if your vehicle has a four-cylinder engine. With a V6 or V8, it could take two to three hours. The cost to replace spark plugs will also vary by vehicle, as some spark plugs designs are more expensive but are designed to last longer.

Which spark plug do you remove first? ›

First things first, What you need to do before attempting any work on a vehicle is to remove the positive terminal cable, so that you are not at risk of an accidental shock. This can be done using an adjustable wrench to match the nut. Next step is to label each of your spark plug wires.

Can you change 1 spark plug at a time? ›

If you aren't replacing the wires, make sure you change one spark plug at a time so you can be sure you connect the right plug to the right wire. If you are replacing the wires, look through your vehicle repair guide for a wiring diagram.

What are common mistakes when changing spark plugs? ›

Two Common Mistakes While Changing Sparkplugs

Possibly the two most common mistakes made with handling sparkplugs---aside from installing the wrong type for your engine---is damaging the spark plug cables by pulling on them; and, misapplying dielectric connector grease to your plugs.

What are the symptoms of too much spark plug gap? ›

Consequences of Incorrect Spark Plug Gap

If the spark plug gap is incorrectly set, it can lead to engine issues. The customer may experience loss of power, misfires, spark plug fouling, increased plug wear, or poor gas mileage.

How do you check spark plugs without removing them? ›

Crank the Ignition and Inspect for Spark Have someone turn the ignition so that you can inspect the plug. Make sure the spark is strong and blue in color. If the spark is weak and dull orange, it's likely that it's not strong enough to ignite the engine's air and fuel mixture.

Is it bad to drive 30 miles with a bad spark plug? ›

Your vehicle may continue to run even if you have bad spark plugs. However, continued operation with bad plugs can cause internal engine damage not to mention the loss of horsepower, poor fuel economy, and overall poor performance.

Should I replace ignition coils with spark plugs? ›

Worn spark plugs can cause unnecessary load on the coils and there is often some overlap in the labor required to replace both components. If your ignition coils failed as a result of worn-out spark plugs, it is wise to replace both components at the same time.

Can you replace only a few spark plugs? ›

Is it OK to replace one spark plug at a time? Even if your vehicle runs nickel electrode spark plugs, replacing one or two at a time will cause potential running issues, so replacing all when there is an issue with one is the best course of action. As the saying goes, 'Do it once, do it right'.

How often should ignition coils be replaced? ›

They used to be replaced around the 30,000 mile mark. This has been extended to between 60,000 and 105,000 miles. However, you should consult your manual for the manufacturer guidelines for your vehicle or feel free to contact your local auto repair shop.

Does AutoZone do spark plugs? ›

When the time comes to replace your plugs, AutoZone has high-quality products to make your repair job a breeze. We stock custom-fit spark plugs and glow plugs for your specific vehicle, ensuring a simple repair process.

Why is replacing spark plugs so expensive? ›

Since replacing the plugs is no longer an annual service, most vehicle manufacturers stopped making them accessible. Today, replacing the plugs may require removing the intake manifold and a lot of other work to reach them.

What does a tune up consist of? ›

Generally, a tune-up consists of checking the engine for parts that need cleaning, fixing, or replacing. Common areas under inspection include filters, spark plugs, belts and hoses, car fluids, rotors, and distributor caps. Many of these only require a visual inspection or a simple test.

What does a misfire feel like? ›

A misfire feels like your engine suddenly loses power. You might experience a hesitation when you try to accelerate the vehicle. If it feels like your vehicle is accelerating slower than usual or if it feels rough, the cause could be an engine misfire.

How many turns for a new spark plug? ›

Seat the new spark plug by hand; turning it clockwise at least two full turns. Once you have the spark plug snug, use the torque wrench with extension and spark plug socket to tighten it turning the plug clockwise.

How much does it cost to replace 6 coils and spark plugs? ›

Repair Cost

The average cost to replace ignition coil(s) and spark plug(s) in 2021 was $387.

Which spark plug is most important? ›

Copper is an excellent conductor of electricity – better than any other type of material used in spark plugs – and also transfers heat faster.

Can you use WD-40 on spark plugs? ›

Using WD-40 to repel water from spark plugs, distributors, alternators, and batteries is a good way to prevent corrosion and keep moisture away. You can also use it to ease the removal of spark plugs, especially if there is any rust or corrosion.

How long does it take a mechanic to change spark plugs? ›

How much time will it take for spark plug replacement? On average, replacing the spark plugs of a four-cylinder engine will take around one hour. Either you can approach a professional who will also check the spark plug wires. But it will also add a labor cost to the spark plug costs.

What a normal spark plug color or appearance should look like? ›

Normal Condition

If the firing end of a spark plug is brown or light gray, the condition can be judged to be good and the spark plug is functioning optimally.

What does a weak spark plug spark look like? ›

A good spark will be blue-white and will be plainly visible in daylight. If a good spark is present, the problem is probably not in the ignition system. Check the fuel system and/or stark timing. Weak sparks are orange or red and may be hard to see in daylight.

How do you test a spark plug to see if it works? ›

Attach a spark plug tester to the ignition wire. At the other end, the tester connects to the spark plug as it sits in your engine. Turn the ignition ON. Start the engine, or simply turn it over if it won't start, and watch the transparent sides of the tester.

What color is a weak spark? ›

The strength of the spark is revealed in the color. A red or yellow spark is weak and probably will not spark in the cylinder. A blue or white spark is strong and has enough voltage to fight across the spark plug gap even under pressure within the cylinder.

What do spark plugs look like when running lean? ›

If the engine is running to lean, the spark plug can look greyish white. If the engine is running to rich the spark plug will look black, sometimes a glossy black if it's really rich.

How long do spark plugs last? ›

Ordinary copper and nickel spark plugs should be changed every 30,000 to 50,000 miles, unless otherwise suggested by the spark plug manufacturer. Platinum and iridium spark plugs tend to last longer than standard copper and nickel spark plugs. Expect to change them every 60,000 to 150,000 miles unless otherwise noted.

What happens when spark plugs 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.

What are common spark plug mistakes? ›

Two Common Mistakes While Changing Sparkplugs

Possibly the two most common mistakes made with handling sparkplugs---aside from installing the wrong type for your engine---is damaging the spark plug cables by pulling on them; and, misapplying dielectric connector grease to your plugs.


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