Top 7 Bad Spark Plug Symptoms: What to Watch Out For?

Is your car running slower than usual? Does your engine misfire every time you press down the accelerator? These are some of the most common bad spark plug symptoms. If you ignore these signs, it may result in minor issues such as MAF sensor malfunction or major problems like a damaged catalytic converter down the line.  

Read on as we delve into the symptoms of bad spark plugs. Knowing these symptoms will help you spot faulty spark plugs early.  

What Do Spark Plugs Do?

Spark plugs are located inside the engine’s cylinders. They fire a high-voltage spark to ignite the air-fuel mixture.  This spark starts combustion and pushes the pistons down. That movement creates the power your engine needs to move the car.

Without spark plugs, your engine can’t start properly, won’t run smoothly, and won’t be able to power your car as it should.

Why are Faulty Spark Plugs a Problem?

Before we get started with the symptoms of bad spark plugs, let’s see why it’s important to catch this issue.

Faulty spark plugs affect how your car runs. They can reduce fuel efficiency and cause your engine to perform poorly. Leave them unchecked, and you risk engine misfires, which can also damage your car’s catalytic converter.

Misfires also send raw fuel into the exhaust, increasing wear on pistons and contaminating your engine oil over time. That means more friction, faster damage, and a repair bill that’s anything but friendly. It doesn’t stop there—bad plugs can raise harmful emissions and reduce engine life with every drive. 

Therefore, it is crucial to identify faulty spark plugs promptly. The symptoms of bad spark plugs shared in the next section will help you do exactly that!

7 Common Bad Spark Plug Symptoms to Watch Out For

Spark plugs seldom fail instantly. In fact, it can take anywhere from a few days to a few weeks before they start showing any signs of trouble. However, it is essential to recognise these signs as early as possible. It’ll save you a lot of trouble down the road. 

So, here are the seven symptoms of faulty spark plugs that you shouldn’t ignore.

1. Engine Misfires

If your car jerks, hesitates, or stutters when you accelerate, a misfire could be to blame. Now, there can be many causes of an engine misfire. However, the most common culprit to blame in such an instance is a faulty spark plug.

We know that spark plugs are responsible for igniting the air-fuel mixture properly. They do so by producing high-voltage sparks. Therefore, when they fail to produce sparks as they should, it results in uneven combustion of the air-fuel mixture.

This uneven combustion leads to jerks at the engine’s output shaft, often known as engine misfires. So, whenever you feel those, make sure the first thing you inspect is your spark plugs.

2. Rough Idling

When your car is parked but the engine is still running, it should sound smooth and steady. If you hear shaking, sputtering, or uneven noises while standing still, that’s a warning sign. One common reason for that is a failing spark plug. 

Even when not driving, the engine needs small, regular sparks to keep running properly. If a spark plug is dirty or damaged, it disrupts the engine’s normal rhythm and causes it to shake.. 

This shaking puts extra strain on critical engine parts over time. If your car doesn’t sound right while waiting at a signal, checking the spark plugs is a smart move. 

3. Trouble Starting the Car

Spark plugs play an important role when it comes to starting the engine. A weak spark plug is unable to produce the initial spark required to start the engine.  This results in longer cranking times or even failed starts.

Cold weather makes the situation worse because engines need more energy to start. Similarly, worn or fouled spark plugs can’t deliver the spark needed under higher loads such as when towing a carvan. Sometimes, repeated starting attempts can even drain your battery faster.

If your car hesitates to start, don’t overlook the condition of your spark plugs. Timely replacement will ensure that you avoid getting stranded unexpectedly.

4. Poor Fuel Economy

An efficient engine burns fuel cleanly and completely with every ignition cycle. However, faulty spark plugs disrupt this process by causing weak or incomplete combustion. This forces the engine to inject more fuel to compensate for lost power.

You’ll notice your petrol expenses rising even if you drive the same routes. That’s because the engine may run slightly richer, wasting even more fuel. 

If you want to stay fuel-efficient, checking the spark plugs is an easy first step. Good spark plugs help restore normal combustion and reduce unnecessary fuel consumption.

5. Reduced Acceleration

Acceleration requires quick, strong ignition to deliver sudden power to the wheels. When spark plugs weaken, the combustion lag affects how fast the engine responds. You might feel your car is hesitating before speeding up. This hesitation becomes more obvious when climbing hills or overtaking.

Sometimes, it can even be dangerous if you can’t accelerate quickly enough in traffic. A worn spark plug delays the power transfer from engine to wheels. Restoring crisp acceleration often comes down to replacing the worn plugs.

6. Illuminated Engine Management Light

Modern cars have sensors that monitor engine performance in real-time. If any of these sensors malfunction, an engine management light will come on. Faulty spark plugs can cause misfires that may lead to MAP, MAF, and various other sensors sending incorrect readings to the ECU.

Even a minor misfire across one cylinder can light up the dashboard warning. At first, you may not feel major differences while driving. But internal stress builds up with each incomplete combustion cycle.

However, getting to the root cause of an engine management light isn’t that simple. There are various issues that can trigger it. Therefore, getting a diagnostic scan is the way to go in such a scenario. It will reveal if fouled spark plugs are the underlying problem.

7. Knocking or Ping Sounds

When spark plugs malfunction, ignition can happen too early or too late. This mistimed combustion causes pressure waves that sound like knocks or pings.

Left unchecked, this creates stress on pistons, valves, and cylinder walls. Continuous knocking can wear out these parts and lower your engine’s lifespan.

Using high-quality fuel can help temporarily, but spark plug replacement is the real fix. If you hear knocking, act quickly to protect your engine’s vital components.

What Happens if You Ignore Bad Spark Plugs?

Ignoring bad spark plugs may seem harmless at first, but it can lead to serious engine problems down the line. It can not only leave you stranded on the roadside but also result in major maintenance bills.

The issue is that misfires from faulty plugs can overheat your catalytic converter, potentially causing permanent damage. Now we all know how expensive it can be to replace a catalytic converter. 

Another issue is that faulty spark plugs can also cause raw fuel to enter the exhaust. This can increase emissions and reduce your vehicle’s overall performance over time. It may also lead to your car failing its emission test that is a mandatory requirement during an MOT in the UK.

Also, worn spark plugs force your ignition coils to work harder. It shortens their lifespan and leads to more repairs you didn’t plan for. 

This is why we always say that routine maintenance is way cheaper than repairs. So, change your spark plugs before this small issue becomes a costly breakdown.

Frequently Asked Questions (FAQs)

1. Is it safe to drive with bad spark plug symptoms?

It is possible to drive with faulty spark plugs, but it is neither safe nor recommended. Faulty spark plugs can cause engine misfires, reducing performance and increasing fuel consumption.

Over time, misfires can damage the catalytic converter, one of the most expensive parts to replace. Driving with bad spark plugs also puts stress on ignition coils and other components. The longer you delay replacement, the higher the repair cost could become.

2. How much does it cost to replace spark plugs in the UK?

In the UK, spark plug replacement typically costs between £100 and £250. However, it can vary depending upon your car’s make, model, and the mechanic’s labour rates.

Most cars have one spark plug per cylinder. For example, a four-cylinder engine needs four spark plugs, while a V8 engine needs eight. Some advanced engines may even use two plugs per cylinder for better combustion. So, firstly the replacement cost varies with the number of spark plugs your car uses. Next, labour makes up a large portion of the cost, especially in models where spark plugs are harder to access.

Replacing spark plugs in cars of different brands also show price differences. Replacing spark plugs in a Ford, for instance, averages £126.69, while for an Audi, it’s around £186.10.

Even within the same brand, prices vary. For example, a Ford Puma costs around £180.29, while a Ford Focus would usually cost less. So, the replacement cost of spark plugs can greatly vary.

This is why before you replace your car’s spark plug, check if selling makes more sense. See what your car is worth with Exchange My Car. Compare offers from specialist buyers and choose the highest offer you can get from our network. Sell any car hassle-free in the UK with Exchange My Car!

3. What is the average lifespan of spark plugs?

The lifespan of spark plugs usually depends on their base material. Standard copper spark plugs typically last around 30,000 miles whereas platinum and iridium spark plugs can last between 60,000 and 100,000 miles. 

However, harsh driving conditions, oil leaks, or poor engine maintenance can reduce their life. Always check your car’s manual or consult a mechanic to confirm the recommended replacement interval.

4. What causes spark plugs to go bad?

Spark plugs will eventually wear out overtime, but certain conditions can accelerate their failure. Here are the key causes and how it may happen:

• Carbon buildup: Incomplete combustion leads to black, sooty deposits on the spark plug tip. This often occurs when the engine runs too rich or the fuel injectors malfunction.
• Oil contamination: Oil leaking into the combustion chamber coats the spark plug, preventing proper ignition. This is usually caused by worn piston rings or valve guides letting oil slip past the seals.
• Overheating: Engine overheating or using a spark plug with the wrong heat range can cause electrode burn. This often occurs when cooling systems fail or if the engine frequently runs at high loads.
• Incorrect gap size: A gap that’s too wide or narrow disrupts the spark, causing incomplete combustion. Poor installation or wear over time can cause the gap to change.
• Poor-quality fuel: Low-grade fuel doesn’t burn cleanly, which creates residue and buildup on the plug tip. This is common with fuel containing high levels of ethanol or impurities.

5. Can bad spark plugs affect transmission?

Bad spark plugs do not directly damage the transmission, but they can affect its performance. Faulty plugs cause engine misfires, hesitation, and inconsistent power delivery. These issues confuse the transmission, leading to rough or delayed shifting. 

If you drive an automatic car, your transmission relies on smooth engine performance to shift correctly. So, while spark plugs and transmission aren’t connected, one can influence the other’s performance.

Wrapping Up: Bad Spark Plug Symptoms

Bad spark plugs don’t just affect how your car drives—they can lead to misfires, rough starts, and higher fuel costs over time. If you continue to ignore the signs, you risk damaging expensive components, such as your catalytic converter or ignition coils.

Replacing bad spark plugs on time helps keep your engine running smoothly and allows you to get more mileage out of every tank. It also boosts your car’s resale value, especially if you plan to upgrade or sell it in the near future. 

If you’re ready to sell your car now, we can help. Exchange My Car connects you with the best car buyers in the UK. Enjoy free collection and fast payment while selling your car through us. Sell your car with Exchange My Car now!

Visit our blog to stay informed on tips about car maintenance, car selling, and car buying in the UK.