A spark plug delivers the electric spark that ignites the air-fuel mixture in each cylinder. That tiny spark, thousands of times a minute, is what keeps a gasoline engine running.
Animated: how a Spark Plug actually works
🔧 How It Works, Step by Step
1
Receive high voltage
The ignition coil sends tens of thousands of volts down to the spark plug at the right instant.
2
Jump the gap
The voltage arcs across the small gap between the center and ground electrodes.
3
Ignite the mixture
That spark lights the compressed air-fuel charge, starting the power stroke.
4
Repeat precisely
The plug fires once per cycle for its cylinder, perfectly timed to engine position.
🧩 The Key Parts
Center electrode
Carries the high voltage down to the tip where the spark forms.
Ground electrode
Provides the target the spark jumps to across the gap.
Insulator
The ceramic body that contains the voltage and manages heat.
Gap
The precise space the spark bridges to ignite the mixture.
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The 50 most common check engine codes with likely cause and DIY fix cost. Sent once.
🩺 Signs of a Failing Spark Plug
Misfires and rough idle
Hard starting
Hesitation on acceleration
Poor fuel economy
Check engine light for misfire
⚠️ Common Problems
Fouled plugs
Oil, carbon, or fuel deposits coat the tip and prevent a clean spark, causing misfires.
Worn electrodes
Over many miles the gap widens as the electrodes erode, making the spark weak or intermittent.
Cracked insulator
A cracked ceramic lets voltage leak away instead of sparking, causing a dead cylinder.
💰 Cost to Fix
$100-$400typical range to repair or replace, parts and labor
❓ FAQ
How often should spark plugs be replaced?
Copper plugs last around 30,000 miles while iridium and platinum plugs can go 60,000 to 100,000 miles or more.
What are signs of bad spark plugs?
Misfires, rough idle, hard starting, hesitation, poor fuel economy, and a check engine light are common signs.
Can I replace spark plugs myself?
On many engines it is a straightforward job with basic tools, though some require removing intake parts or coils to reach the plugs.