Spark Plug Health Checker
Check your vehicle for common signs of cracked spark plugs. Select all symptoms you're experiencing to get an assessment of potential issues.
Symptoms to Check
If your car is sputtering, struggling to start, or losing power on the highway, you might be dealing with something simple-and something expensive if ignored. One of the most common but overlooked issues is a cracked spark plug. It doesn’t always scream for attention, but when it does, your engine pays the price. You don’t need a garage full of tools to spot the signs. With a little attention, you can catch a cracked spark plug before it turns into a bigger, costlier problem.
What a cracked spark plug looks like
A cracked spark plug isn’t always obvious. The crack might be a hairline fracture on the ceramic insulator-the white part that surrounds the metal center electrode. It’s often hidden inside the engine, so you won’t see it unless you pull the plug. But you don’t need to guess. There are clear symptoms that point to damage.
When the ceramic insulator cracks, it loses its ability to contain the high-voltage spark. Instead of jumping cleanly from the center electrode to the ground electrode, the electricity leaks through the crack. This causes misfires, poor fuel economy, and sometimes even engine damage. In cold weather, moisture can seep into the crack and short the spark entirely. That’s why many people notice problems first thing in the morning.
Sign #1: Engine misfires
One of the first signs you’ll notice is the engine misfiring. You might feel a jerking sensation when accelerating, or the car might shake at idle. The check engine light may flash-this isn’t a gentle reminder. A flashing light means the engine is misfiring badly enough to risk damaging the catalytic converter.
Why does this happen? A cracked spark plug can’t deliver a consistent spark. Sometimes it fires, sometimes it doesn’t. That inconsistency throws off the engine’s timing. Modern cars will detect this and log a P0300 code (random misfire) or a specific cylinder code like P0302 (cylinder 2 misfire). A scan tool will show you which cylinder is failing.
Sign #2: Hard starting, especially in cold weather
If your car used to turn over in a second and now takes three or four tries, especially on a frosty morning, it’s not just the battery. A cracked spark plug can cause hard starting because the spark is leaking or weak. Cold air is denser, so the fuel-air mixture is harder to ignite. A healthy spark plug can handle it. A cracked one can’t.
Some drivers think it’s the fuel pump or the starter motor. But if you’ve replaced those and the problem remains, check the spark plugs. I’ve seen people spend £200 on a new starter only to find out the real issue was a cracked plug in cylinder 3.
Sign #3: Poor fuel economy
When spark plugs misfire, unburned fuel exits the cylinder and gets dumped into the exhaust. That’s wasted money. You might notice your fuel tank empties faster than usual-maybe 10-15% worse than normal. If you’ve been tracking your mpg and it’s dropped suddenly, don’t assume it’s traffic or tires. Spark plugs are a cheap fix that could save you £50 a month on fuel.
One driver in Bristol tracked his fuel use for six months. After replacing cracked spark plugs, his average jumped from 32 mpg to 39 mpg. That’s £300 a year saved on petrol alone.
Sign #4: Rough idle or stalling
When the engine is idling-stopped at a traffic light, parked with the engine running-it should be smooth. If it’s shaking, vibrating, or dying out, that’s a red flag. A cracked spark plug can cause intermittent misfires even at low RPM. The engine doesn’t have enough power to compensate, so it stumbles.
Some people think this is a dirty throttle body or a vacuum leak. Those are possible too, but spark plugs are easier to check. And if you’ve got multiple symptoms-misfires, hard starting, rough idle-it’s almost certainly the plugs.
Sign #5: Visible damage on the plug
Here’s the simplest test: remove the spark plug and look at it. You’ll need a spark plug socket, an extension, and a ratchet. Don’t rush this. Let the engine cool down first-hot metal can burn you.
Once you pull the plug, inspect the ceramic insulator. Look for white or brownish cracks, especially near the base where it meets the metal shell. Sometimes the crack is tiny, like a hairline fracture. Other times, you’ll see a chunk missing. Don’t ignore small cracks. Even a micro-fracture can let moisture in and cause failure.
Also check the electrodes. If they’re melted, eroded, or covered in thick carbon, the plug is worn out. But if the ceramic is cracked, it’s not just worn-it’s broken. Replace it immediately.
Sign #6: Fuel smell from the exhaust
If you’re standing behind your car and smell raw petrol, that’s not normal. It means unburned fuel is passing through the engine and coming out the tailpipe. A cracked spark plug is one of the most common causes. The spark isn’t igniting the mixture properly, so fuel escapes unchanged.
This isn’t just wasteful-it’s dangerous. Unburned fuel can ignite in the catalytic converter, causing it to overheat and melt. Replacing a catalytic converter costs £800 or more. A set of spark plugs? Under £50.
Sign #7: Increased emissions and failed MOT
In the UK, your car must pass an MOT test every year. One of the key checks is exhaust emissions. A cracked spark plug causes higher hydrocarbon (HC) and carbon monoxide (CO) levels. If your car failed its MOT for emissions, and you’ve already checked the air filter and fuel system, look at the spark plugs.
One mechanic in Bristol told me he sees this pattern every autumn: cars failing MOT after winter, and the root cause is always cracked or worn plugs. It’s not the exhaust system-it’s the ignition.
What causes spark plugs to crack?
Cracks don’t just happen. They’re caused by:
- Over-tightening during installation. Spark plugs need precise torque. Too much force cracks the ceramic.
- Thermal shock. If you pour cold water on a hot engine, or drive through deep water right after a long run, the sudden temperature change can crack the insulator.
- Low-quality plugs. Cheap spark plugs use thinner ceramic or poor manufacturing. Stick to brands like NGK, Bosch, or Denso.
- Age. Even if you haven’t driven hard, spark plugs degrade over time. Most last 30,000 to 100,000 miles, depending on type.
What to do if you find a cracked spark plug
Replace it. Don’t wait. Don’t try to glue it. Don’t keep driving on it. A cracked plug can damage your cylinder head, valves, or catalytic converter. The longer you wait, the more expensive it gets.
Replace all the spark plugs at once-even if only one is cracked. They wear together. If one failed, the others are close behind. Use the exact type your car manufacturer recommends. Check your owner’s manual for the correct heat range and thread size.
When installing new plugs, use a torque wrench. Don’t guess. Over-tightening is the #1 cause of cracks. Most spark plugs need 15-20 Nm of torque. Too much, and you break it. Too little, and it won’t seal right.
How often should you check your spark plugs?
Check them every 30,000 miles or every two years, whichever comes first. If you drive in stop-and-go traffic, in cold climates, or use low-quality fuel, check them more often. Keep a log. Write down the date you last changed them. That way, you’ll know when it’s time.
It’s a five-minute job with the right tools. You don’t need to be a mechanic. Just follow the steps: disconnect the battery, remove the coil packs (if your car has them), pull the plug, inspect it, install the new one with the right torque, and reconnect.
Prevention is cheaper than repair. A £40 set of spark plugs can save you £800 in catalytic converter costs. That’s not a gamble. That’s smart maintenance.
Can a cracked spark plug cause a car to not start at all?
Yes, especially if the crack is severe or if moisture has entered the plug. A cracked spark plug can cause a complete misfire in that cylinder. If your car has a 4-cylinder engine and one plug is dead, it might still start-but it’ll run poorly. If two or more plugs are cracked, the engine may not start at all because it can’t generate enough power to turn over properly.
Can you drive with a cracked spark plug?
You can, but you shouldn’t. Driving with a cracked spark plug risks damaging your catalytic converter, which can cost over £800 to replace. It also increases emissions, reduces fuel economy, and can cause sudden engine misfires while driving-dangerous on highways. Get it fixed within a few days.
Do all spark plugs look the same when cracked?
No. Cracks can be tiny hairline fractures near the base, or they can be large chunks missing from the ceramic. Sometimes the crack is hidden under dirt or carbon buildup. Always clean the plug with a wire brush before inspecting. Use a flashlight to look closely at the ceramic insulator-especially where it meets the metal shell.
Can a spark plug crack from using the wrong type?
Yes. Spark plugs come in different heat ranges. If you use a plug that’s too hot for your engine, it can overheat and crack the ceramic. If it’s too cold, it can foul with carbon. Always use the type recommended in your owner’s manual. Using the wrong plug can cause cracks even if you install it correctly.
Is it better to replace all spark plugs at once?
Yes. Spark plugs wear at similar rates. If one has cracked, the others are likely close to failure. Replacing them all at once ensures even performance, smoother engine operation, and avoids having to do the job again in a few months. It’s more cost-effective in the long run.
Final tip: Keep your spark plugs in good shape
Spark plugs are one of the cheapest parts to maintain-and one of the most important. They’re the spark that makes your engine run. When they fail, everything else suffers. Don’t wait for the check engine light to flash. Check them during routine maintenance. Keep a record. Use quality parts. And never over-tighten.
A small inspection now can save you hundreds-and keep your car running smoothly for years to come.
