Bad Spark Plug Symptoms: 8 Signs It's Time to Replace Them

The most common bad spark plug symptoms are misfires, a rough or shaky idle, hard starting, hesitation when you accelerate, worse fuel economy and a check engine light. The catch is that ignition coils, fuel injectors and vacuum leaks cause exactly the same symptoms, so read the trouble codes and look at the plugs before you buy parts.

What you need

• An OBD-II code reader. Cars sold in the US since the 1996 model year have the OBD-II port, as do gasoline (petrol) cars sold in the EU since 2001. It's usually under the dashboard near the steering column.
• A spark plug socket, ratchet and extension to pull a plug. The 16 mm magnetic spark plug socket with swivel fits the most common plug size and holds the plug as you lift it out. Some engines use 14 mm plugs, so check yours first.
• A wire-type gap gauge to measure the old plug
• Your owner's manual, for the plug type, gap and replacement interval
• A flashlight

8 signs of bad spark plugs

1. The check engine light is on or flashing

Your engine computer watches for misfires. When it sees enough of them, it turns on the check engine light and stores a code: P0300 for random or multiple-cylinder misfires, and P0301, P0302 and so on for a misfire in cylinder 1, cylinder 2 and onward. A steady light means get it checked soon. A flashing light means a misfire severe enough to damage the catalytic converter, because unburned fuel overheats it. Ease off, avoid hard acceleration and get the car checked as soon as you can.

2. Misfires you can feel

A misfire feels like a stumble, jerk or brief loss of power, often with an uneven exhaust sound. A worn plug needs more voltage to fire, and that demand rises with cylinder pressure, so misfires usually show up first under load: accelerating hard, climbing a hill or towing.

3. Rough or shaky idle

If one cylinder fires weakly or not at all, the engine runs unevenly. You feel it as vibration through the seat and steering wheel, most noticeably when you're stopped in gear. The tachometer needle may bounce slightly.

4. Hard starting

Worn or fouled plugs struggle to light the mixture during cranking, especially on a cold morning, so the engine cranks longer before it catches. A weak battery, a tired starter or a fuel delivery problem can do the same, so if the engine cranks slowly, check the battery first.

5. Hesitation or sluggish acceleration

If the car bogs down, stutters or feels flat when you press the accelerator, a weak spark is one possible reason. It's the same load-related problem as sign 2, just milder.

6. Worse fuel economy

Every misfire sends fuel out of the exhaust without doing any work, so an engine with bad plugs uses more fuel for the same driving. Track it over several fill-ups: miles driven divided by gallons added, or liters per 100 km. Be realistic, though. New plugs restore economy you lost to worn or misfiring plugs. They won't improve an engine that was already running well.

7. Stalling

Badly fouled or failing plugs can make the engine stumble enough to stall at idle or as you come to a stop. Stalling has plenty of other causes, so treat it as a reason to read the codes, not as proof the plugs are bad.

8. The plugs are overdue or look worn

Sometimes the clearest sign is the service record. If your plugs are past the interval in your owner's manual, they're due, however the car feels. Long-life iridium and platinum plugs wear slowly, so an engine can feel fine right up until misfires start.

Other problems that cause the same symptoms

Spark plugs are only one cause of misfires and rough running. Rule out the usual suspects before you spend money:

Possible cause	Typical clues
Ignition coil or plug wire	A misfire code for one cylinder that moves to another cylinder when you swap coils
Fuel injector	A misfire that stays in the same cylinder after you swap the plug and coil
Vacuum leak	Rough idle that smooths out at higher rpm, a hissing sound, lean codes such as P0171 or P0174
Low compression	A misfire that stays in one cylinder with a good plug, coil and injector. It needs a compression test.
Fuel supply or sensors	Misfires spread across several cylinders, often with lean or sensor codes

How to check whether your spark plugs are bad

1. Read the codes. Plug in the code reader with the ignition on and write down every code before you clear anything. Some scan tools also show live misfire counts for each cylinder. If you don't own a reader, some auto parts stores will read codes for you.
2. Find the cylinder. Cylinder numbering differs between engines, so check a service manual or diagram for yours before you start swapping parts.
3. Do a swap test. On engines with one coil per cylinder, swap the coil from the misfiring cylinder with a neighbor, clear the codes and drive. If the misfire moves with the coil, the coil is faulty: see how to replace an ignition coil. If it stays put, the plug is the next suspect, then the injector.
4. Pull the plug. Let the engine cool completely, blow dirt away from the plug well (wear eye protection), and remove the plug with a spark plug socket.
5. Read the plug using the list below.
6. Measure the gap. If an old plug's gap is well over your engine's spec, the electrodes have worn away.

What a bad spark plug looks like

• Normal: light tan or gray deposits and crisp electrode edges.
• Worn: a rounded, eroded center electrode and a gap noticeably wider than spec. Replace the set.
• Carbon fouled: dry, sooty black deposits. Common causes are a rich mixture, a weak spark, lots of short trips or a plug that's too cold for the engine.
• Oil fouled: wet, black and oily. Oil is getting into that cylinder, for example past worn piston rings or valve seals. New plugs only hide this for a while.
• Overheated: a white, blistered insulator or melted electrodes. Possible causes include the wrong heat range, a lean mixture, ignition timing problems or a plug that wasn't tightened properly. It needs diagnosing, not just a new plug.
• Damaged: a cracked insulator or a bent electrode. Replace it and find out why it happened.

Replace one plug or all of them?

Replace them as a set. They've all covered the same miles and worn at much the same rate, so if one is worn out, the others aren't far behind. Use the exact part number your manual or the plug maker's catalog lists. A different heat range can cause fouling or overheating. Our step-by-step guide on how to change spark plugs covers the whole job on a cold engine.

Mistakes to avoid

• Buying plugs before reading codes. A quick scan can save you from replacing parts that were fine.
• Driving on with a flashing check engine light. A catalytic converter costs far more than plugs or a coil.
• Clearing codes before you write them down. You lose the clue that tells you which cylinder to look at.
• Ignoring what the old plugs show. Oil, heavy soot or blistering points to a problem that new plugs won't fix.
• Expecting new plugs to fix everything. If the symptoms come back quickly, the cause is somewhere else.

When to call a pro

See a mechanic if the misfire continues after new plugs and a coil swap test, if the plugs are wet with oil or coolant, if the check engine light keeps flashing, or if a plug is seized or breaks. The same goes for engines where some plugs sit under the intake manifold, which makes the job much bigger.

FAQ

Are bad spark plug symptoms worse when accelerating?

Often, yes. A worn plug needs more voltage to fire, and the demand rises with cylinder pressure, so hesitation and misfires usually appear first when you accelerate hard or climb a hill.

Can bad spark plugs cause a car not to start?

They can if the plugs are badly fouled or very worn, but it's not the most likely cause. A weak battery, a faulty starter, a fuel problem or an immobilizer fault are more common reasons for a no-start. Check those first.

Can bad spark plugs cause bad gas mileage?

Yes. Misfiring cylinders waste fuel, so worn plugs can lower your mileage. Replacing them gets back what you lost, but new plugs won't raise mpg beyond what a healthy engine already achieves.

Can bad spark plugs cause white smoke?

Not directly. Thin white vapor on a cold morning is usually just condensation. Thick white smoke that keeps coming can mean coolant is burning, blue smoke usually means oil, and black smoke usually means too much fuel. Persistent smoke of any color needs checking.

How do I know if it's the spark plug or the ignition coil?

Swap the coil from the misfiring cylinder with one from a healthy cylinder and drive. If the misfire code follows the coil, replace the coil. If it stays with the cylinder, check the plug next.

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