Spark Plug Replacement Calculator
Spark Plug Replacement Calculator
Your car sputters on startup. It hesitates when you press the gas. Maybe it’s just a bit rougher than usual. You might think it’s the fuel, the battery, or even the weather. But more often than not, it’s your spark plugs. They’re small, cheap, and easy to ignore-until your engine starts acting up. Spark plugs don’t last forever. And if you wait too long to replace them, you’ll pay for it in fuel costs, performance, and even engine damage.
What spark plugs actually do
Spark plugs create the tiny electric spark that ignites the air-fuel mixture in your engine’s cylinders. Without that spark, nothing happens. Your engine won’t start. Or if it does, it runs poorly. Modern cars use either standard copper, platinum, or iridium spark plugs. Iridium plugs last the longest-often up to 100,000 miles. Copper ones might need replacing every 20,000 to 30,000 miles. The type in your car depends on the make, model, and year. Check your owner’s manual for the recommended replacement interval.
Signs you need new spark plugs
- Hard starting - Your car takes longer than usual to turn over. You crank the key, hear the engine groan, and it finally catches after a few tries. This isn’t just a weak battery. It’s the spark plugs struggling to ignite the mix.
- Engine misfires - You feel a sudden jerk or stumble when accelerating. The engine sounds uneven, like it’s skipping beats. Misfires happen when one or more plugs fail to fire properly. Left unchecked, this can damage your catalytic converter.
- Reduced fuel economy - You’re filling up more often. Your car used to get 40 mpg on the motorway. Now it’s down to 32. Worn spark plugs don’t burn fuel cleanly. That means wasted gas and more trips to the pump.
- Sluggish acceleration - You press the pedal, but the car doesn’t respond like it used to. There’s a delay, a lag. It’s not the transmission. It’s the engine not getting the right spark at the right time.
- Check Engine light - If the light comes on, don’t ignore it. A code like P0300 (random misfire) or P0301-P0308 (cylinder-specific misfire) often points directly to bad spark plugs.
What worn spark plugs look like
If you’re comfortable popping the hood, you can check your plugs yourself. Remove one with a spark plug socket and extension. Look at the electrode-the metal tip sticking out. A healthy plug has a light brown or greyish colour. A worn one shows signs like:
- Black, sooty deposits - This means the air-fuel mixture is too rich. Could be a bad injector or sensor, but often it’s just old plugs.
- White or blistered electrode - Too much heat. Could mean the plug is wrong for your engine, or you’ve been driving hard for too long.
- Cracked porcelain - A visible crack in the white ceramic part. This plug is done. Replace it immediately.
- Worn or rounded electrode - The sharp edges are gone. The gap between the centre and side electrode is too wide. That’s the #1 sign of age.
Use a gap tool to measure the gap. Most modern cars need a gap between 0.028 and 0.032 inches. If it’s wider than 0.040 inches, the plug is worn out.
How often should you replace spark plugs?
There’s no one-size-fits-all answer. But here’s a general guide based on plug type:
| Plug Type | Expected Lifespan | Best For |
|---|---|---|
| Copper | 20,000-30,000 miles | Older cars, budget replacements |
| Platinum | 60,000-80,000 miles | Most mid-range vehicles |
| Iridium | 80,000-100,000 miles | Modern cars, high-performance engines |
These are estimates. If you drive mostly short trips in the city-especially in cold weather-your plugs wear faster. Short trips don’t let the engine reach full operating temperature, so carbon builds up. If you tow heavy loads or drive aggressively, they wear out quicker too.
What happens if you don’t replace them
Ignoring bad spark plugs doesn’t just hurt performance. It can cost you thousands.
- Increased fuel consumption - A single misfiring cylinder can drop your fuel economy by 15-20%. That’s £15-£20 extra per month on fuel.
- Catalytic converter damage - Unburned fuel from misfires flows into the exhaust. It overheats the converter, melting the internal honeycomb. Replacement costs £800-£1,500 in the UK.
- Engine damage - In extreme cases, misfires cause pressure imbalances. Valves, pistons, or even the head gasket can suffer. Repairs? Often more than the value of the car.
When to replace spark plugs - even if nothing’s wrong
Don’t wait for symptoms. If your car is approaching the manufacturer’s recommended interval, replace them proactively. It’s cheaper than fixing what breaks later. For example, if your manual says 100,000 miles and you’re at 95,000, plan for replacement soon. Especially if you’re planning a long trip or live in a place with cold winters like Bristol.
Also, if you’ve bought a used car and don’t know when the plugs were last changed, assume they’re worn. It’s a £50-£150 job to replace them all. Much better than being stranded on the M5 with a misfiring engine.
How to choose the right spark plugs
Not all spark plugs are created equal. Using the wrong type can cause problems. Always match:
- Thread size and reach - The plug must screw in properly without touching the piston.
- Heat range - Too cold? Carbon builds up. Too hot? Pre-ignition or melting. Your manual specifies the correct heat range.
- Electrode material - Stick to the OEM recommendation unless you’re upgrading for performance. Iridium is usually the best choice for longevity.
Brands like NGK, Denso, Bosch, and Champion are reliable. Avoid cheap no-name plugs. They often have inconsistent gaps or poor materials.
DIY or professional?
Replacing spark plugs is one of the few engine jobs you can do yourself with basic tools: a socket wrench, extension, spark plug socket, and gap tool. Most cars take under an hour. But some engines-especially those with tight spaces or coil-on-plug designs-can be tricky. If you’re unsure, or your car has a turbocharged engine, get a mechanic to do it.
Even if you do it yourself, always torque the plugs correctly. Overtightening cracks the porcelain. Undertightening causes leaks and misfires. Use a torque wrench if you can. The typical torque range is 15-20 lb-ft for most cars.
Final tip: Keep a record
Write down the date and mileage when you replace your spark plugs. Stick it in your service book or take a photo. That way, you won’t forget next time. And if you sell the car, you can prove you kept up with maintenance. Buyers notice.
Can bad spark plugs cause the check engine light to come on?
Yes. A misfire caused by worn or faulty spark plugs is one of the most common reasons the check engine light turns on. Codes like P0300 (random misfire) or P0301-P0308 (specific cylinder misfire) directly point to spark plug issues. Replacing the plugs often clears the code-but always scan the system again after replacement to confirm.
Do I need to replace all spark plugs at once?
Always. Even if only one plug looks bad, the others are likely close to failing. Replacing just one can cause uneven performance because the new plug fires hotter or more reliably than the old ones. This stresses the engine and can lead to more problems. Replace the full set for smooth, balanced operation.
Can I clean old spark plugs instead of replacing them?
Cleaning worn spark plugs isn’t recommended. You might remove surface carbon, but you can’t restore a worn electrode or fix a cracked insulator. The gap will still be too wide, and the metal will be thin. A cleaned plug might work temporarily, but it won’t last. It’s cheaper and safer to replace them.
How much does it cost to replace spark plugs in the UK?
Parts alone range from £20 for basic copper plugs to £80 for premium iridium sets. Labour at a garage typically costs £50-£100, depending on engine access. Some cars take longer to reach the plugs, especially V6 or turbocharged engines, which can push labour to £150. DIY, you can do it for under £100 total.
Do spark plugs affect emissions?
Yes. Worn spark plugs cause incomplete combustion, which increases hydrocarbons and carbon monoxide in your exhaust. This can cause your car to fail its MOT. A clean, properly gapped spark plug helps your engine burn fuel completely, reducing emissions and keeping you legal.
If you’ve noticed any of these symptoms, don’t wait. Spark plugs are a simple fix that can save you money, stress, and expensive repairs down the line. Check them. Replace them. Drive better.
