📋 Quick Facts
Time
15-30 minutes
Difficulty
Easy
Tools
4 tools
Cost to DIY
$20-40
Spark plugs are the cheapest, easiest, and most diagnostic part of the ignition system. Pull one out, look at it, gap it, and test for spark - the whole process takes 15 minutes per cylinder.
🛠 What You'll Need
- Spark plug socket with rubber insert (shop spark plug sockets on Amazon)
- Spark plug gap tool, wire or coin style (shop spark plug gap tools on Amazon)
- Spark tester (inline or adjustable-gap) (shop spark testers on Amazon)
- Digital multimeter (shop a digital multimeter on Amazon)
Product links above are Amazon affiliate links. AmpAuto earns a small commission at no extra cost to you.
⚠ When NOT to DIY thisNever pull a spark plug from a hot engine. The aluminum cylinder head shrinks around the plug threads as it cools, and yanking a hot plug can strip the threads. Wait until the engine is at ambient temperature.
🎯 Expected Readings (Pass/Fail Reference)
| Spark plug gap (gas) | 0.028" - 0.060" (varies by car - check shop manual or door sticker) |
| Plug tip color (healthy) | Light tan to light gray |
| Black, sooty | Running rich (bad O2, MAF, or injector leak) |
| White, blistered | Running lean or detonation (vacuum leak, weak fuel pump) |
| Oil-fouled | Worn rings, leaking valve seals, or PCV problem |
| Plug resistance (resistor plugs) | 3,000 - 7,500 ohms typical |
Numbers are typical. Always cross-check against your factory service manual for the exact spec.
📝 Step-by-Step Test Procedure
- Wait for the engine to be ambient temperatureCold engine. Aluminum heads + hot plugs = stripped threads. Patience pays here.
- Disconnect the coil and pull one plug at a timeUnbolt the coil, set it aside, and use the spark plug socket with rubber insert to remove the plug. Keep it pointed up so debris does not fall into the cylinder.
- Read the plug color and conditionLight tan/gray = healthy. Black sooty = running rich. White/blistered = running lean or detonation. Wet with fuel = no-fire (bad coil, bad plug). Oil-coated = piston/ring/valve seal issue.
- Check the gap with a wire gaugeLook up the spec for your engine (door sticker, owner's manual, or under the hood label). Slide the wire gauge through the gap - it should drag slightly. Adjust by gently bending the ground electrode, never the center electrode.
- Resistance test (resistor plugs only)Set multimeter to 20 k-ohms. One probe on the terminal nut at the top, the other on the threaded body of the plug. Should read 3,000 - 7,500 ohms. Open circuit = internally cracked plug.
- Spark test with the plug back inPlug it onto an inline spark tester, ground the tester to the engine. Crank for 2 seconds - bright blue spark across at least 1/4" (6 mm) gap. Weak orange/yellow = bad coil, fouled plug, or low compression.
- Cylinder color comparisonAfter pulling all plugs, lay them in order. All four/six/eight should look similar. One that is dramatically darker, lighter, or oily points to a specific cylinder problem.
- Replace if past life or visibly wornIridium plugs: 100,000 miles. Platinum: 60,000-100,000. Copper: 30,000. If the electrode is rounded off, swap them. shop NGK Iridium spark plugs on Amazon.
✅ Pass / Fail Criteria
✅ PASS
Tan/gray color, gap within 0.005" of spec, 3-7.5 k-ohm resistance, bright blue spark at 1/4" gap, threads clean
❌ FAIL
Black/oily/white tip, gap outside spec by more than 0.010", infinity resistance, weak yellow spark, or worn-off ground electrode
🔧 If It Fails - What To Do Next
Replace the spark plugs. Most engines take 4, 6, or 8 plugs at $4-15 each. Always replace as a complete set. If one plug shows a unique color (oil-fouled, sooty), investigate that cylinder for the root cause. See How long do spark plugs last? and How often to change spark plugs.
🔗 Related Guides
❓ Frequently Asked Questions
How do I know my spark plug gap?
Check the under-hood emissions sticker, owner's manual, or your service manual. Common ranges: 0.028" for many turbos, 0.040" for typical V8s, 0.060" for some older engines.
What does a wet spark plug mean?
Fuel without ignition. Either the coil is not firing, the plug is bad, or compression is too low to ignite the mixture.
Can a spark plug be bad even if it looks fine?
Yes - internal resistor failure or a hairline ceramic crack only shows up under load. Use the spark tester as the final check.
How long do iridium spark plugs last?
100,000 miles on most modern engines. Some manufacturers spec 120,000.
Should I gap iridium plugs?
Most are pre-gapped from the factory. Verify with a gap tool but only adjust if more than 0.005" off - the tip is brittle.
Can a worn spark plug cause a misfire?
Yes. As the gap widens, the coil has to work harder. Eventually the spark fails under load and you get a P0301-P0308 code.