How to Replace Spark Plugs

When to replace spark plugs

Service interval depends on plug type:

• Copper plugs: 30,000 miles
• Platinum plugs: 60,000–80,000 miles
• Iridium plugs: 80,000–100,000+ miles

Symptoms of bad plugs: rough idle, hesitation under acceleration, drop in fuel economy, hard starts, check engine light with P0300-series misfire codes. (Pull the code with our OBD2 guide to confirm.)

What you need

• Spark plug socket — 5/8" or 13/16" depending on plug. Has a rubber insert that grips the porcelain so the plug doesn't fall out.
• Ratchet with 6" extension — reaches into deep plug wells.
• Torque wrench — non-negotiable. Over-torqued plugs crack porcelain. Under-torqued plugs blow out.
• Gap gauge — coin-style or wire-style. Both work.
• Dielectric grease — inside coil boots to prevent moisture and stuck boots.
• Anti-seize — single dab on threads. NOT a generous coating.
• Compressed air or vacuum — clear plug wells before removing old plugs.
• The right plugs. Look up the part number for your exact year/make/model/engine. Don't substitute. Don't mix brands.

Reading the old plug

Before throwing the old plugs away, look at the electrode tip. The color tells you a lot:

• Light tan/grey: Engine running properly. Normal wear.
• Black, sooty: Running rich. Could be a faulty O2 sensor, MAF, or stuck injector.
• Black, oily: Oil burning past the rings or valve seals. Bigger problem.
• White, blistered: Running lean or overheating. Check fuel pressure, vacuum leaks.
• Mechanical damage (cracked porcelain, melted electrode): Pre-ignition or detonation. Stop driving and investigate.

Step-by-step (compressed)

1. Cold engine, battery disconnected. Wait until the engine has been off for at least 2 hours. Disconnect negative battery terminal.
2. Access the plugs. Remove engine cover. Label plug wires by cylinder if present.
3. One at a time. Disconnect coil pack or plug wire. Vacuum out the plug well. Loosen the plug with the socket, spin out by hand.
4. Gap the new plug. Verify gap against spec with a gap gauge. Adjust if needed.
5. Anti-seize + dielectric grease. Dab anti-seize on threads (one drop, not a coating). Apply dielectric grease in the boot.
6. Hand-thread first. Three full turns before any tool. If it binds, back out and try again — cross-threading the head is expensive.
7. Torque to spec. Snug, then torque-wrench to the exact figure for your engine.
8. Reinstall coil/wire. Push boot until it clicks. Bolt down coil to spec.
9. Repeat for remaining cylinders. Work one at a time so you can't mix up coils or wires.
10. Reconnect battery, start engine. Should idle smoothly. Listen for misfires.
11. Log it in Trackara. Date, mileage, plug brand and part number. Sets the next reminder.

Common mistakes that cost real money

• Removing plugs from a hot engine. Stripped aluminum threads. Repair cost: $200–$1,000.
• Over-torquing. Cracked porcelain or stripped threads. Use the torque wrench.
• Too much anti-seize. Affects torque reading. One drop on the threads is enough.
• Skipping the gap check. Modern iridium plugs ship close to spec but not always exact. Always verify.
• Mixing brands or heat ranges. Causes uneven combustion. Run all-same plugs.
• Forgetting which coil goes where. Each cylinder has a specific coil. Work one at a time or label everything.