📋 Quick Facts
Time
15-min check, 30–90 min replace
Difficulty
Easy–Medium
Tools
5
Cost
$30–$200
Spark plug replacement intervals have stretched dramatically over the past 30 years thanks to platinum and iridium tip materials. But "long-life" plugs aren't maintenance-free - they slowly erode, gap widens, and ignition voltage demand climbs. Replacing them on schedule (or slightly before) is one of the cheapest ways to protect fuel economy and prevent misfire codes.
🛠 What You'll Need
- Spark plug socket (5/8" or 9/16" usually)
- Torque wrench (15–25 ft-lb range)
- Anti-seize compound (sparingly, on aluminum heads)
- Dielectric grease for boots
- 6" extension and ratchet
⚠ When NOT to DIY thisOn Ford 5.4L Triton (2004–2008), spark plugs are notorious for breaking off in the head - research the TSB procedure (engine warm, PB Blaster, partial-loosen-and-retighten cycles) or pay a shop $400 to do it right. A broken plug can mean a $1,500 head removal.
📝 Step-by-Step Instructions
- Identify your plug typeCheck the owner's manual or look at the part you're replacing. Copper = $2 each, 30k mile life. Platinum = $5 each, 60k. Iridium / Double-iridium = $10–$15 each, 100k–120k.
- Look up the interval for your engineOwner's manual is gospel. Common intervals: Honda/Toyota iridium 105k, GM 5.3L iridium 100k, Ford modular V8 100k, Subaru 100k, BMW double-iridium 60k–100k.
- Watch for early-replacement symptomsRough idle, MPG drop, P0300/P0301-P0308 codes, slow starts, or hesitation under load - replace early if any are present, even if you're only at 70% of the interval.
- Pull a plug at the halfway pointAt 50k on a 100k plug, pull one plug and inspect. Tan color = healthy. Black/oily = burning oil. White = running lean. Use the inspection to catch other problems.
- Replace plugs in a cool engineNEVER pull plugs hot - the aluminum threads expand and you can strip the head. Engine cool (overnight or several hours).
- Torque to spec, no exceptionsAluminum heads (most modern engines) demand exact torque - typically 13–25 ft-lb. Over-torque cracks the head; under-torque blows the plug out.
- Replace coils too if they're original at 100k+Coil packs degrade alongside plugs. If you're at 100k+ and one coil has misfired, swap all of them while you're in there. ~$30 each.
🔗 Related Guides
❓ Frequently Asked Questions
What happens if I don't replace spark plugs?
Misfires, MPG loss, and on Ford modular V8s, plug seizure that can cost $1,500+ to fix. Replace on schedule - it's cheap insurance.
Can I mix plug brands?
No. Use the OEM-spec plug only - gap, heat range, and electrode geometry vary. NGK iridium IX or Denso iridium long-life are safe OE-equivalent options for most cars.
Do I need to gap iridium plugs?
Most iridium plugs are pre-gapped from the factory. Don't bend the fine-wire electrode - you can break it. Verify with a wire feeler gauge but adjust only if clearly wrong.
Does anti-seize on plug threads cause problems?
On modern plated plugs (most NGK/Denso), don't use it - the coating already prevents seizure and over-torquing risk goes up. On old uncoated plugs in aluminum heads, a tiny dab is fine.
How do I know if my coil packs are bad too?
Pull codes - P030X tells you which cylinder. Swap the suspect coil to a different cylinder; if the misfire follows, it's the coil. Bad coils run $30–$80 each on most cars.
Can I drive with one bad spark plug?
Briefly - a single-cylinder misfire dumps unburned fuel into the catalytic converter and can destroy a $1,200 cat in 50 miles. Fix within days.