Bad spark plugs cause rough idle, misfires, hard starting, hesitation when accelerating and worse fuel economy. Spark plugs wear gradually as the electrode gap grows, so the symptoms often creep in slowly. Most modern platinum or iridium plugs last somewhere between 60,000 and 100,000 miles, while traditional copper plugs are often replaced around every 30,000 miles; your owner manual lists the interval for your engine.
Bad spark plug symptoms
- Rough idle and shaking at a stop; see rough idle.
- Misfires and a flashing check engine light under load, with codes such as P0300 or a cylinder-specific code like P0301.
- Hard starting, especially in cold or damp weather.
- Hesitation or jerking when accelerating; see car jerks when accelerating.
- Lower fuel economy and reduced power.
When to replace spark plugs
| Plug type | Typical replacement interval |
|---|---|
| Copper core | About 20,000 to 30,000 miles |
| Platinum or double platinum | About 60,000 to 100,000 miles |
| Iridium | About 60,000 to 120,000 miles |
These are general ranges; always use the interval and plug type listed for your engine. Boosted and high-performance engines often need colder plugs, smaller gaps and shorter intervals.
Spark plug gap
The gap is the distance between the center and ground electrodes. Use the gap specified for your engine, which is often printed on the underhood emissions label or in the service information. Many iridium and platinum plugs come pre-gapped; check them with a wire gauge and adjust only the ground strap, never by prying against the fine center electrode. Engines with forced induction usually need a smaller gap than the same engine without boost.
Reading old spark plugs
- Light tan or gray deposits: normal.
- Black, dry soot: running rich; see P0172.
- Wet, oily deposits: oil getting into the cylinder past rings or valve seals; see piston rings and exhaust smoke colors.
- White, blistered or melted electrodes: overheating, from a lean mixture, the wrong heat range or detonation; see engine knocking.
Replacing spark plugs
- Let the engine cool, especially with aluminum cylinder heads.
- Remove one coil and plug at a time to avoid mixing up cylinders.
- Blow debris out of the plug well before removing the plug.
- Thread new plugs by hand first, then torque to specification; follow the plug maker guidance on anti-seize.
- Inspect coil boots for cracks or oil, and replace them if damaged; see bad ignition coil symptoms.
If new plugs foul quickly with oil or a cylinder shows low compression, the engine itself is worn; see compression test and compare engines by code on our engines page.
More guides: spark plug wires · check engine light flashing · car hesitates when accelerating · how often to change spark plugs · iridium vs platinum spark plugs · outboard spark plugs.
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