What Are The Common Symptoms Of Failing Spark Plugs?

If your car has faulty spark plugs, you’ll notice some of these symptoms:

1. Check Engine Light Turns On

A worn spark plug can cause your Check Engine Light to turn on or keep flashing.

However, a faulty spark plug isn’t the only reason why your Check Engine light illuminates. It can also light up due to spark plug wire or coil pack issues.

Since faulty spark plugs are a common cause of Check Engine light activation, it doesn’t hurt to start by diagnosing your spark plug. You may encounter signs of mechanical damage or fuel fouling (spark plugs getting flooded with fuel).

2. Reduced Fuel Efficiency

Bad spark plugs increase your vehicle’s fuel consumption because they can’t efficiently burn the airfuel mixture going into your engine. 

In fact, spark plug failure can reduce your car’s fuel efficiency (a.k.a. fuel economy or gas mileage) by around 30%.

As a result, you’ll have to make more frequent trips to the gas station and shell more money on gasoline.

3. Rough Engine Idle

Under normal conditions, your engine will sound steady and smooth.

But if your vehicle has a faulty spark plug, you’ll hear pining, rattling, or knock-like noises coming from the engine. These are signs that your vehicle’s ECU (Electronic Control Unit) is struggling to compensate for the loss of power due to bad spark plugs.

4. Car Struggles To Start

Your spark plugs provide the spark needed to ignite the air/fuel mixture in the combustion chamber.

Old and worn spark plugs will struggle to create this spark and power up your engine. And the engine likely stalls when you try to turn on the ignition system. 

However, if your car won’t start, the problem could be also lie with your car battery.

5. Engine Hesitates During Acceleration

Another common symptom indicative of worn spark plugs is engine hesitation.

You step on the accelerator pedal, but your vehicle isn’t as responsive as it used to be and accelerates sluggishly. In other words, it’ll feel like your vehicle has lost its ability to get up and go.

6. Exhaust Emissions Smell Like Gasoline

Unburnt gasoline will reach the vehicle’s exhaust system when your spark plugs don’t ignite the air/fuel mixture properly in the combustion chamber. Consequently, your vehicle’s tailpipe emissions will start to smell like gasoline.

Your exhaust may also smell like gas if you’ve got a clogged air filter, a leaky fuel injector, or a defective mass airflow sensor.

But troubleshooting your spark plugs is a good starting point.

How Much Does Spark Plug Replacement Cost?

On average, spark plug replacement can cost anywhere between $100 and $500.

We can break down the total cost into:

• Spark plug cost: while conventional copper spark plugs cost $2-$10/piece, premium iridium spark plugs can cost up to $100/piece.
• Labor cost: you can expect to pay between $40 and $350 as labor charges, depending on your car’s engine type.

The cost of replacing your spark plugs will also vary depending on the make and model of your vehicle, your location, and the brand of spark plugs you use.

How Urgent Is A Spark Plug Replacement?

If a spark plug or two is faulty, your engine will have to run on fewer cylinders and can’t generate enough power. This condition, where one or more of your engine’s cylinders don’t produce power, is known as an engine misfire.

An engine misfire can damage your catalytic converter, sap engine performance, reduce fuel efficiency, and cause irreversible engine damage.

That’s not all.

A bad spark plug also fails to generate enough spark to fire up your engine. And as you repeatedly crank the engine hoping for a start, you may drain the battery dead.

In short, replace your faulty spark plugs ASAP to prevent engine misfires and start-up troubles.

4 Spark Plug Replacement FAQs

Let’s explore some more about spark plug replacement through these four commonly asked questions:

1. Why Do Spark Plugs Need Replacing?

Every time a spark jumps the gap between the center and ground electrodes, it burns off small amounts of metal from each electrode. And this gap continuously increases as your vehicles spark plugs constantly fire during engine operation.

With time, the spark plug gap grows so much that the spark can longer make the jump.

In other words, your spark plug fails. 

You’ll then experience dips in fuel economy, slow acceleration, and rough idling.

2. How Often Will I Need To Replace Spark Plugs?

Your spark plug’s lifespan can range between 20,000 miles and 100,000 miles (for a long life spark plug).

It varies depending on the:

• Spark plug metal type
• Spark plug manufacturer
• Spark plug design
• Car’s make and model

Consult your car owner’s manual for more clarity on when to change spark plugs.

However, if you’re looking for a general rule, most manufacturers recommend changing spark plugs every 30,000 miles.

3. How Do I Replace A Bad Spark Plug?

Spark plug replacement can be pretty challenging on some vehicles. 

You may need to remove the engine’s upper plenum or intake manifold before reaching the spark plugs. Moreover, you can have difficulty accessing and removing components like the ignition coil or spark plug wire.

Not just that.

If you do a bad job with the spark plug replacement, you can lead to expensive engine repairs.

That’s why it’s best to let a professional mechanic at an auto repair and maintenance shop change your vehicles spark plugs.

That said, here’s what usually happens during spark plug replacement: 

Your mechanic will first gather the tools/parts required — new spark plugs, spark plug socket, ratchet, torque wrench, spark plug gapping tool, anti-seize lube, dielectric grease, etc.

After that, they’ll:

• Remove the engine cover (if your car has one).
• Disconnect the ignition coil connector.
• Remove the old spark plug using an appropriate spark plug socket.
• Clean the spark plug hole to remove debris that can damage the cylinder head threads.
• Gap your new spark plug according to manufacturer requirements if it’s not pre-gapped.
• Apply a thin layer of dielectric grease to the spark plug boot.
• Install the new spark plug.
• Tighten the new plug with a torque wrench based on the manufacturer’s torque specification.
• Reinstall the ignition coil connector and engine cover.
• Fire up the car and ensure everything works as expected after installing the new plug.

4. Do I Need To Replace All The Spark Plugs?

Ideally, you should.

Changing spark plugs lets you prevent situations like engine misfire, bad gas mileage, etc.

If you’re replacing only those spark plugs that have gone bad, each spark plug in your engine will have a different replacement cycle. Consequently, you’ll need to spend more effort and time with repairs whenever you encounter a spark plug failure.

Moreover, your engine works best when all the spark plugs have similar performance levels.

For instance, if two spark plugs in your six-cylinder engine have one level of performance and the rest have a different level, it can cause the engine to go out of sync.

In short, replace all your spark plugs at once to ensure a consistent level of performance.