NGK lists rough idling, misfiring, hard starting and worse fuel economy among the symptoms of bad spark plugs, and explains that a misfire happens when the spark takes the easiest path instead of jumping the gap. Carbon fouling, worn ignition parts, too large a gap and damaged plugs are among the causes NGK names.
Likely causes
A misfire code that names one cylinder (for example P0302 for cylinder 2) narrows the search a lot.
| Cause | Clues | DIY or shop |
|---|---|---|
| Worn or fouled spark plugs | Past the replacement interval; random or multiple-cylinder misfire | DIY on many 4-cylinder engines |
| Failing ignition coil | Misfire stays on one cylinder; moves if you swap coils | DIY on many engines |
| Wrong plug or gap | Misfire after recent plug change | DIY: use the plug and gap your manual specifies |
| Vacuum leak | Hissing, lean codes, worse at idle | DIY inspection; repair varies |
| Fuel injector or fuel pressure | Misfire on one cylinder that is not the coil or plug | Shop |
| Mechanical problem | Low compression on one cylinder; ticking or knocking | Shop |
From NGK published spark plug guidance and EPA OBD information; not a diagnosis of your vehicle.
What you can check yourself
- Get the codes read for free at AutoZone or Advance Auto Parts to see whether the misfire is on one cylinder or random.
- Check your owner’s manual for the spark plug replacement interval; many iridium plugs are specified for about 100,000 miles.
- On engines with coil-on-plug ignition, swapping the suspect cylinder’s coil with a neighbor and rechecking shows whether the misfire follows the coil.
- Use the exact plug type and gap your manual lists; NGK warns that a non-resistor plug where a resistor plug is required can cause rough idle and misfires.
When to go to a shop
If new plugs and a coil swap do not fix it, or the misfire comes with knocking, smoke or overheating, get a professional diagnosis. Long-term misfiring can overheat and damage the catalytic converter, an expensive repair.
Safety: work on a cool engine, disconnect the coil connectors before removing coils, and torque spark plugs to the specification so threads are not damaged.
Spark plug prices and DIY cost
The NGK Iridium IX and Autolite Iridium XP plugs we checked were the same price at both stores. See spark plug prices and the DIY vs shop calculator.
Frequently asked questions
Yes. NGK lists rough idling and misfiring among the symptoms of bad spark plugs.Can bad spark plugs cause a rough idle?
Short distances with a mild, steady misfire may be possible, but a flashing check engine light means a severe misfire. Minimize driving and get it checked to protect the catalytic converter.Is it safe to drive with an engine misfire?
Yes, plugs are normally replaced as a set so every cylinder fires the same.Should I replace all spark plugs at once?
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We compare auto parts on published specs, warranty terms and dated store prices, and we link every store policy to its official page. We do not run a lab, so we never claim hands-on tests we did not do. About us · How we compare · Editorial policy