Spark Plug Diagnosis – Read Your Spark Plugs to Find Engine Faults

For almost any problem with a crucial car part or system, there are going to be recognizable symptoms.

Following on from this, the condition of specific car parts can be a good indicator of parts of your engine’s health.

Certain components in vehicles are easy to troubleshoot. Others can be much more of a challenge.

Spark plugs usually fall into the first category, and that’s what this article is going to discuss: Using spark plug diagnosis to look for possible issues with your engine.

One used and one new spark plug side by side against a green background

You don’t have to take the whole engine apart to find your spark plugs, making the inspection and the use of them for diagnosing many car troubles simple.

Below, we’ve compiled a user-friendly spark plug diagnosis guide. You’ll learn about 11 of the most common complications they suffer from. This includes descriptions of symptoms and suggested actions to take.

We’ll also warn you about the consequences of each fault if you don’t address it and allow them to get worse.

Common Faults With Spark Plugs and Their Causes

Here’s a quick list of the common spark plug faults:

  • Ash deposits.
  • Carbon deposits.
  • Detonation.
  • Gap bridging.
  • High-speed glazing.
  • Insulator cracked or broken.
  • Lead fouling.
  • Mechanical damage.
  • Oil deposits.
  • Pre-ignition.
  • Too hot.
  • Worn electrode.

What follows in the article from here, is the first section telling you what spark plug’s look like when they’re normal and functional.

The subsequent troubleshooting sections on spark plug fault diagnosis, are listed in alphabetical order.

They’re not ranked according to severity, frequency, or any other criteria. These variables can be subjective, and we’d rather keep it objective.

Normal Spark Plugs

We’re defining normal spark plugs as those that look and are working exactly as they should.

These deliver high-voltage discharges to your ignition system. The resulting spark is what ignites the fuel to run your engine.

Watch this brief animation to see how they work:


General Appearance

Appearance can vary slightly based on the type you bought, but all types are built using the same concepts.

A porcelain shell conceals an inner electrode. This runs from the terminal (tip) all the way down to the firing end. The base is threaded so it can be screwed into your cylinder head.

A healthy unit will be gray, tan or pale brown at the nose or tip. The electrode may be mildly eroded, but nothing more severe than that. It should still retain a square edge.

There aren’t any symptoms or troubles to speak of here. Engine performance is normal; everything is working as it should.

Recommendations and Actions to Take

Replace normal spark plugs as per your vehicle manufacturer’s recommendations. This will usually be every 30,000 miles or so, but more details can be found in our guide on how often to change spark plugs.

Ash Deposits

Ash deposits can be the result of a range of issues from minor to serious:

  • Incorrectly tightened plugs.
  • Low coolant or oil level.
  • Lean air to fuel ratio (excess air).
  • Leaking oil.
  • Incompatible oil viscosity.

Appearance, Symptoms, and Trouble it May Cause

You’ll find these deposits covering the electrodes and insulator tip. They’re usually an off-white color with a cinder-like consistency, like cigarette ash.

The symptoms of this fault can manifest as:

  • Engine overheating.
  • Decreased engine performance.
  • Trouble starting the engine.
  • Oil pressure warning light.
  • Coolant warning light.
  • The smell of burning oil.
  • Rough idling.

If you’re lucky, you have a spark plug that hasn’t been screwed in correctly – check our article on how tight spark plugs should be. The other causes of ash deposits can be bad news for your vehicle in the long term.

Running low on coolant and oil is asking for trouble. Engine overheating will wear out moving mechanisms faster. If you push your motor too far in this state, it could fail.

The wrong ratio of fuel to air can negatively impact your fuel economy.

An inappropriate viscosity of oil for your car translates into poor lubrication. This heightens the risk of friction damage and jamming.

Recommendations and Actions to Take

Investigate potential culprits one by one, according to the symptoms you have. For instance, the smell of burning oil likely signals a leak.

Check for engine damage afterward. Replace your spark plugs once you’ve completed the repair.

Carbon Deposits

This type of deposit might be the result of one or more of the following:

  • Fuel mixture too rich (not enough air).
  • Dirty air filter.
  • Frequent bouts of short-distance driving.
  • Long stretches of driving at low speeds.
  • Unit is too cold.

Appearance, Symptoms, and Trouble it May Cause

Carbon deposits are soot- black and soft to the touch. They’re the result of low operating temperatures.

They’re often accompanied by the following symptoms:

  • Trouble starting the engine, especially when it’s cold.
  • Misfiring.
  • Rough idling.
  • Check engine light appears.
  • Sluggish acceleration.
  • Lower fuel mileage.

A motor that’s running rough due to complications is working overtime. This additional effort translates to unnecessary strain on crucial mechanisms.

An overly rich fuel mixture will cost you in gas money. The surplus fuel won’t positively influence performance either. Instead, you’ll find difficulty accelerating and switching the ignition on.

Recommendations and Actions to Take

Check your air filter to see if it has to be replaced. This may solve the lack of air in your fuel mixture. The soiled plug will also have to be changed.

Next, consider correcting your driving practices. Try to take your vehicle for longer drives every so often rather than exclusively short trips.

Take a trip on the highway every so often. If you’re not comfortable doing this, have someone help you. Always driving at low speeds isn’t good for your motor.

Detonation Signs

The signs of detonation are not to be taken lightly. This term refers to when spontaneous combustion happens after the spark plug has fired.

This problem could be caused by:

  • Incompatible fuel octane.
  • Lean fuel mixture (too much air).
  • Overheating engine.
  • Advanced ignition timing.
  • Malfunctioning exhaust gas recirculation (EGR) system.

Appearance, Symptoms, and Trouble it May Cause

The insulator nose will be noticeably damaged. You may see that it’s cracked, split or otherwise broken.

The major symptom of detonation is a knocking or pinging sound from your engine. Other common signs can include:

  • Lowered engine performance.
  • Rough idling with heavy vibrations.
  • Trouble starting the motor.
  • Smoking exhaust.

Aside from cracking your plugs, this can also destroy your pistons. Other components can also suffer from abnormal combustion. Detonation can result in engine failure if not addressed.

Recommendations and Actions to Take

Take swift action to find out what’s going on. Check that your EGR system is working and that your fuel mixture and octane is suitable for your vehicle.

Review your ignition timing if necessary. If it’s too advanced, the abnormal combustion will keep happening.

Remedy an overheating engine by checking all feasible causes. Do you have enough oil and coolant? Could you have a blown head gasket or a leaky radiator?

Make sure you dispose of the detonated units, changing them out for new ones. Your motor will have to be examined afterward for internal damage.

Gap Bridging

Gap bridging is a consequence of deposits collecting in the combustion chamber. The partially solid compounds liquefy when you put a high torque load on the engine.

The newly liquefied material then collects and solidifies between the electrodes of the spark plug. Hence the name—the deposits form a bridge over the gap.

This occurrence could be due to one, or a combination, of:

Appearance, Symptoms, and Trouble it May Cause

The space between the center and ground electrode will be bridged by a clump of blackened deposit. The insulator nose may also be discolored.

Your vehicle may be experiencing one or more of the following:

  • Misfiring engine.
  • Rough idling.
  • Increased emissions.
  • Decreased fuel mileage.
  • Sluggish acceleration.
  • Detonation (knocking or pinging).

As you already know, detonation can lead to damage and even failure. The deposits can also interfere with the electrodes, prompting them to short circuit. The connecting cylinder will misfire and eventually cease firing at all.

A greater load on your motor will stress the whole system. Ignoring the presence of deposits can also cause blockages to form in your engine, damaging it.

Recommendations and Actions to Take

You may want to have your combustion chamber thoroughly cleaned out. If you often drive through heavy-traffic areas, you may want to alter your route, if you can.

Choose oil and fuel brands that are rich in detergent agents. These additives will break down buildup and reduce the chances of this happening again.

You’ll also want to give your vehicle a thorough once-over. If you’ve ignored the symptoms for a while, other issues could have escalated.

For mild deposits, you may be able to clean the spark plug and use it again. If the buildup is heavy, it’s best to switch it out with a new one. Fortunately, spark plugs aren’t very expensive.

High-Speed Glazing

This type of glazing is caused by sudden and extreme acceleration. The temperature in the combustion chamber rises abruptly, melting normal deposits on the spark plug.

These then harden, forming a glaze. This is sort of like the automotive equivalent of cooking sugar, but unappetizing.

Appearance, Symptoms, and Trouble it May Cause

The insulator nose will appear shiny. The glaze may have a yellowish, greenish or brownish tinge to it.

The glaze acts as a conductor for electricity. In turn, that can cause the affected device to short-circuit when it heats up. This problem manifests as engine misfires when you

accelerate hard or drive fast.

The hard glaze can’t be cleaned off the unit. That means it’s ready for the trash, even if you just bought it.

Erratic engine performance is dangerous, especially when you’re going fast. If your motor is consistently misfiring, you’ll be a hazard to yourself and others on the road.

Recommendations and Actions to Take

Dispose of the glazed units, switching them out for new ones. You may want to purchase plugs that run colder if you tend to hit the accelerator hard.

If not, practice accelerating gradually as opposed to suddenly. This approach is kinder to your spark plugs and your engine.

Insulator Cracked or Broken

A broken insulator can be one of the signs of detonation. We kept that section separate due to the severity of that particular concern. If you skipped over it, go back and read through. You want to be certain none of the symptoms your vehicle is experiencing could warn of detonation.

Another reason why your insulation may have broken is due to poor installation practices. These are over-tightening and aggressive use of your wrench.

Appearance, Symptoms, and Trouble it May Cause

What it looks like will depend on how bad the break is. The insulator could have a hairline crack or be missing a sizable chunk.

The broken unit won’t be able to spark consistently, causing misfires. If the break is bad enough, it won’t work at all. You’ll be driving with a dead cylinder, which should of course be avoided.

Recommendations and Actions to Take

You can’t fix a broken or cracked insulator. Even if you could, it wouldn’t be worth it. Buy another one to replace the damaged unit. Be gentle with installation this time.

Lead Fouling

Lead-fouled plugs can be the outcome of using fuel with a high content of lead-based additives.

The lead, known as tetraethyllead, serves as an octane booster. Older vehicles may require this type of fuel for lubrication to operate smoothly.

Unfortunately, the lead may form deposits inside the combustion chamber that can result in fouling.

Appearance, Symptoms, and Trouble it May Cause

Lead deposits on the insulator nose can be brown or yellow, with a greenish tinge. The glaze can be light and spotty, or heavy and covering the whole nose.

The deposits can conduct electricity when hot enough, which means they can short circuit your unit. This will typically only occur when you’re driving at high speeds.

It’s preferable for both you and your vehicle to have all your cylinders firing normally. Neglecting to change the broken unit can cause a dead cylinder, which can damage your engine.

Recommendations and Actions to Take

Glazing is impossible to clean off. Buy a new one and get rid of the fouled spark plug. If you’ve been dealing with misfires for a long time, have your motor checked.

You might want to reconsider your fuel or additive choices. Choose one that isn’t as lead-heavy.

Mechanical Damage

This term may sound a little odd to you. Isn’t all spark plug damage mechanical in nature?

In this respect, the term mechanical doesn’t mean automotive. It refers to damage that’s caused by other components. This consists of:

  • Strong vibrations due to careless installation.
  • Internal components hitting the plug (incorrect installation is likely).
  • Debris or a foreign object trapped in the combustion chamber striking the plug.

Appearance, Symptoms, and Trouble it May Cause

The ground electrode, center electrode, or both, may be broken. This includes chips, cracks, bending, or missing pieces.

In this state, it won’t be working consistently. That will lead to an overall decrease in engine performance such as:

  • Slow acceleration.
  • Engine misfires.
  • Poor fuel mileage.

An erratically-behaving engine is frustrating to handle. Plus, decreasing fuel mileage will take a toll on your wallet.

If other parts of your engine are striking the plug, they could be damaged too. For instance, if your pistons are colliding with the plug’s firing end.

A foreign object bouncing around your combustion chamber can spell disaster. It can cause trouble for more than just your plugs, hitting other mechanisms too.

Recommendations and Actions to Take

Plugs that have been physically damaged must be replaced. When you remove them, check to see if it was installed correctly. If not, you know what caused the problem.

If all looks well installation-wise, a foreign object may be at fault. The mystery item may even be a piece of the plug that broke off.

You’ll have to inspect your engine for further damage. The cylinders, pistons and other areas could have been struck by the object too.

Oil Deposits

The one good thing you can say about leaking oil is that it isn’t discreet. If there’s a breach somewhere, you’ll eventually find oil where it shouldn’t be.

Leaking oil is the primary cause of discovering these deposits on your plugs. The oil is likely coming from a failing seal somewhere in your engine. Worn-out valve guides could also be at fault.

Appearance, Symptoms, and Trouble it May Cause

When there is oil on spark plugs, the electrodes and insulator nose will be fouled with oil. It may be soaked or a little wet, depending on how serious the leak is. The oil deposits may also have hardened to a pale brown crust.

Associated symptoms of oil deposits may include:

  • Overheating.
  • Misfires.
  • The vehicle is consuming more oil than normal.
  • A strong odor of burning oil.
  • Blue-black exhaust smoke.
  • Smoking engine compartment.
  • Oil puddles under car.

Lubricant leaks can do grave damage if left unrepaired for long enough. Unusually high operating temperatures are stressful for your motor in general.

A car that’s spewing blue exhaust smoke is also an environmental hazard. You could receive a fine if you drive your vehicle in this condition.

Recommendations and Actions to Take

Uncover where the leak is coming from and plug it up. Once that’s been repaired, clean all the plugs that have oil deposits on them. If they’re soaked in oil, it’s easier to purchase new ones.

Check that your engine hasn’t sustained damage if you’ve been driving with symptoms for a while.


This can be described as over-eager combustion. Fuel and air combust prematurely before the plug delivers a spark.

Possible causes can include:

  • Advanced ignition timing.
  • Imbalanced fuel mixture (too much air or fuel).
  • Low levels of coolant, oil, or both.
  • Sharp edges on components inside the combustion chamber.
  • Overheating spark plug.

Appearance, Symptoms, and Trouble it May Cause

The surface of the electrode will be scorched or melted. It will look deformed, lumpy, and uneven.

Keep an eye out for these symptoms:

  • Misfires.
  • Trouble speeding up.
  • Rough idling.
  • Knocking, warning of detonation.
  • Overheating.
  • Check engine light comes on.

Pre-ignition can be caused by detonation. A telltale knocking from your motor combined with the other symptoms must not be dismissed.

Remember that ongoing detonation and engine failure go hand in hand. The other items on the list aren’t much better. A strained motor means poor performance and increased likelihood of damage.

Recommendations and Actions to Take

Take into account the heat range of your plugs. If it isn’t compatible with your vehicle, it may be running too hot. Either way, the melted units can’t be restored and must be replaced.

If that isn’t the case, go through the list of possible causes above. Begin with what’s easiest (e.g. checking oil and coolant levels) and work your way up.

You’ll want to take swift action, especially if you’re hearing knocking. Don’t let detonation destroy your motor.

Too Hot

Plugs that are too hot are usually caused by:

  • Incompatible heating range for your car.
  • Advanced ignition timing.
  • Lean fuel mixture.
  • Low oil levels.
  • Clog in the coolant system.

Appearance, Symptoms, and Trouble it May Cause

There are trademark signs of heat damage. The insulator will be blistered and white, and the electrode will have begun to erode. Normal deposits will be absent, burned away as soon as they form.

Symptoms can be:

  • Misfiring motor.
  • Overheating.
  • Oil pressure warning light switched on.
  • Rough idling.
  • Hard-to-start engine.
  • Pre-ignition.
  • Reduced fuel economy.

A super-heated spark plug won’t have a very long lifespan. However, this is the least concerning aspect of this problem.

As we touched on in the section above, overheated plugs can induce pre-ignition. Any abnormal combustion is unhealthy for your motor in the long run.

The other causes and symptoms can all lead to a damaged engine. A lack of lubrication, blocked coolant system, or incorrect fuel mixtures aren’t minor issues.

Recommendations and Actions to Take

Investigate to uncover the source of your overheating plugs. First, check if the heat range of the models you bought is compatible. This is the best-case scenario.

If not, go through the rest of the potential issues one by one. Your motor needs to be examined for subsequent damage too.

Once repairs are complete, you’ll be switching the burnt-up units with new ones. They won’t be salvageable.

Worn Electrode

A worn electrode is typically caused by normal wear. It will gradually erode over the course of the plug’s lifetime.

Appearance, Symptoms, and Trouble it May Cause

The electrode will look rough and worn down. It won’t be the same size and shape as when it was brand new.

Symptoms will include frequent misfires. If the spark plug has aged past a certain point, it might not be capable of sparking at all.

Recommendations and Actions to Take

Change the old plug out for a fresh one. In the future, keep track of how many miles you’ve put on your spark plugs to know when they’re past their prime.

Final Thoughts

Spark plug diagnosis can tell us a lot about what’s happening inside our cars. If your engine is behaving unusually, you may want to take a look at them.

You could get a clearer idea of what could be causing the trouble. This will enable you to repair whatever it is before it gets worse and results in more extensive damage.

Do you have any questions for us or diagnosis stories to share? Please leave us a comment below.

From childhood go karting and motocross, to collecting and obsessing over scalextric, matchbox and radio controlled cars, I've always had an obsession with cars. Learning through manuals, books, trial and error, and more knowledgeable family members, I've also enjoyed tinkering with the mechanics and electronics of any vehicles I've owned. Now, over 3 decades later, I've started this site as a place for me to share my knowledge, to teach others how to care for and maintain their vehicles themselves, at home, so they can get the most of their vehicles and save a pretty penny compared to always seeking out professional help.

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