Spark plugs wear out every 60,000-100,000 miles, and when they do, your engine runs rough, wastes gas, and loses power. A shop charges $200-400 for this job. You can do it in your driveway in under an hour for $20-40 in parts.
What You Need
Tools
- Spark plug socket (5/8" or 16mm for most cars) — these have a rubber insert to grip the plug. Spark plug sockets on Amazon →
- Ratchet with extension (3/8" drive, 6" extension) — plugs sit deep in the cylinder head
- Torque wrench — over-tightening cracks the porcelain or strips the threads
- Gap gauge — most modern plugs come pre-gapped, but always verify. Gap gauges on Amazon →
- Dielectric grease — prevents the boot from seizing onto the plug
Parts
- Spark plugs — match the exact part number from your owner's manual. Iridium plugs cost more but last 100K miles. Don't mix brands or heat ranges.
- Anti-seize compound (optional, debated) — some manufacturers say yes, some say no. Check your manual.
Step by Step
Step 1: Work on a Cold Engine
Unlike oil changes, spark plugs should be done cold. Aluminum cylinder heads expand when hot, and removing a plug from a hot aluminum head risks damaging the threads.
Step 2: Remove One Plug at a Time
This is the most important rule. If you pull all the plug wires or coil packs at once, you might mix up the firing order. Work on one cylinder, finish it, then move to the next.
Step 3: Remove the Coil Pack or Wire
On modern cars, each plug has its own coil-on-plug (COP). Press the release tab and pull straight up — don't wiggle side to side. On older cars with plug wires, grab the boot (not the wire) and twist while pulling.
Step 4: Remove the Old Plug
Use your spark plug socket with extension and ratchet. Loosen counterclockwise. Once loose, spin it out by hand. Inspect the old plug — the color and condition tell you a lot about how your engine is running.
Step 5: Check the Gap on the New Plug
Your manual specifies a gap (usually 0.028"-0.044"). Slide the gap gauge between the electrodes. If it's wrong, gently bend the ground electrode to adjust.
Step 6: Install the New Plug
Start by hand. Thread the plug in with your fingers or with the socket (no ratchet) for the first few turns. This prevents cross-threading, which is an expensive fix on aluminum heads. Once finger-tight, torque to spec (usually 12-18 ft-lbs for 14mm plugs).
Step 7: Reinstall the Coil Pack
Apply a tiny dab of dielectric grease inside the boot, then push the coil pack down until it clicks. This grease prevents moisture and makes the next removal easier.
Step 8: Repeat for Each Cylinder
4-cylinder = 4 plugs, V6 = 6 plugs, V8 = 8 plugs. Some V6 engines have the rear bank plugs tucked under the intake manifold — those are harder to reach but the same process.
Common Mistakes to Avoid
- Cross-threading — always start by hand. If it resists, back out and try again.
- Over-tightening — use a torque wrench. Stripped spark plug threads in an aluminum head means a trip to the machine shop.
- Wrong gap — causes misfires, rough idle, or poor fuel economy.
- Wrong heat range — stick with the factory-spec plug. "Performance" plugs rarely improve anything on a stock engine.
How Do You Know They Need Replacing?
- Rough idle or engine vibration
- Poor fuel economy
- Slow acceleration
- Check engine light with misfire codes (P0300-P0308)
- Hard starting, especially in cold weather
Check our Spark Plug Replacement guide for vehicle-specific notes and torque specs.