How to Remove a Stuck or Seized Spark Plug Safely

To remove a stuck spark plug, let the engine go completely cold, fit a snug socket on a straight extension and apply steady hand force. If the plug won't turn, soak the threads with a little penetrating oil, then work it out slowly, turning it back and forth a small amount at a time. Never force a seized plug out of an aluminum cylinder head: if it won't move with reasonable effort, stop, because a snapped plug or stripped threads costs far more than the plug itself.

What you need

• A spark plug socket that fits the hex snugly. A 16 mm magnetic spark plug socket has a thin 1 mm wall for deep plug wells and a magnet to lift the plug out, but some engines use 14 mm plugs, so check yours with our spark plug socket size guide.
• A 3/8-inch drive ratchet and straight extensions (not wobble extensions)
• Penetrating oil, or the solvent your engine's service information specifies
• Compressed air and eye protection
• New plugs and a torque wrench for refitting
• Time. Soaking and slow, patient work are far safer than force.

Why spark plugs get stuck

• Overtightening at the last change, or a plug that went in cross-threaded
• Carbon and corrosion on the threads, especially where they're exposed to combustion
• Plugs left in far beyond their change interval
• Corrosion between the steel plug and an aluminum head. NGK, for example, plates its plug threads to resist corrosion and help them release.
• Engine design. Some engines are known for it: Ford, for example, issued a service bulletin with a special removal procedure for certain 4.6, 5.4 and 6.8 liter three-valve V8s built up to late 2007.

Before you start

• Cold engine only. NGK says to remove plugs only from a cool engine, because metal expands when hot and threads can be damaged. Ford's bulletin for its problem engines calls for the engine to be at room temperature. Overnight is ideal.
• Look for a known procedure. Search the service information for your exact engine. If the carmaker publishes a specific removal method or solvent, use it instead of generic advice.
• Remember what you're working against. Most modern cylinder heads are aluminum, which is much softer than the steel plug. If something gives way under force, it's the threads in the head.
• Skip the heat. You'll find advice online to work on a hot engine or use a torch. Plug makers say cool engine, and a flame near a cylinder head full of wiring, plastic and fuel lines is a fire risk.

How to remove a stuck spark plug step by step

Step 1: Clean the plug well

With the coil or plug wire off, blow out the well with compressed air, wearing eye protection. Grit on top of the plug can jam the threads as the plug comes up, or fall into the cylinder once it's out.

Step 2: Get a perfect socket fit

Seat the socket fully on the hex so it doesn't rock. Use a straight extension and keep everything in line with the plug: side load can crack the ceramic insulator or round the hex. If your socket has a swivel, keep it straight for this job. Put one hand on the ratchet head and the other on the handle.

Step 3: Try steady force

Turn counterclockwise with smooth, increasing pressure: no jerking, no cheater bar and no impact wrench. A plug that's simply tight will crack loose and then turn smoothly, and you can carry on. If it doesn't move with reasonable force, or turns a little and then gets very tight, stop and go to Step 4.

Step 4: Soak the threads

1. Apply a small amount of penetrating oil around the base of the plug, where it meets the head.
2. Don't flood the well. Fluid that seeps past the threads can collect in the cylinder, and Ford's bulletin specifically warns that too much can hydro-lock the engine.
3. Wait at least 15 minutes. A badly seized plug may need several hours, with a top-up in between.

Step 5: Work it back and forth

1. Turn the plug out a small amount, around 1/8 of a turn, or as far as it goes without a big jump in effort.
2. Turn it back in a little, then out a little further. Repeat, gaining a bit more each time.
3. Add penetrating oil as you go. Squeaking and high effort are normal at this stage. Ford's procedure for its problem engines uses the same in-and-out motion.
4. If the effort keeps rising instead of easing, stop, soak again and come back later.

Step 6: Lift it out cleanly

Once the plug turns freely, stop with a few threads still engaged and blow out the well again, so loosened carbon and oil don't drop into the cylinder. Then unscrew it the rest of the way. A magnetic socket brings the plug up with it.

Step 7: Check the threads before fitting a new plug

Look at the old plug's threads. Shiny flakes of aluminum caught in them can mean the threads in the head are damaged. Start the new plug by hand: it should turn freely for several turns. If it binds, stop. A spark plug thread chaser can clear light carbon from the head threads, but it isn't a fix for stripped ones. Then fit and tighten the new plug as described in our guide on how to change spark plugs, to the torque in your manual. Don't add anti-seize unless your manual calls for it: NGK and Denso both advise against it on their plugs.

Troubleshooting: when it still won't come out

• The ratchet clicks but the plug doesn't turn. The direction lever is set to tighten. Flip it.
• The socket slips on the plug. The socket is the wrong size or the hex is rounding. Stop at once. Don't hammer a smaller socket on as you might with a bolt: the ceramic insulator can shatter. This is a job for a shop.
• The plug turns but doesn't come up. The threads in the head are probably stripped. Stop turning, because you're making it worse, and read the next section.
• It's the boot that's stuck, not the plug. Twist the coil or boot back and forth to break the seal before you pull. Our guide on how to replace an ignition coil covers this.
• The plug breaks. Stop. Don't drill it, and don't reach for a screw extractor set: those are made for screws, not a broken plug shell sitting in an aluminum head. Champion warns that spiral-cut extractors can expand a broken shell and lock it in tighter. Broken plugs need the right tool for that engine and someone who has done it before.

Stripped spark plug threads

If the plug came out with aluminum in its threads, or a new plug won't thread in or tighten, the threads in the head are damaged. The usual fix is a thread repair insert: the hole is cut oversize and a steel insert gives the plug new threads. Kits exist that do this with the head still on the engine. It's a proven repair, but it has to go in straight and every metal chip has to be kept out of the cylinder. For most people, this is a job for a shop.

Don't drive with a plug that won't tighten properly. Combustion pressure can blow it out of the head, taking what's left of the threads with it and damaging the coil.

When to call a pro

Get help if the plug won't move after a couple of soak-and-rock sessions, if it keeps getting tighter as it comes out, if the hex starts to round, if the plug breaks, or if you see aluminum on its threads. A shop has the removal tools and thread repair kits for these jobs, and calling one early costs far less than recovering from a snapped plug.

FAQ

Should you remove spark plugs hot or cold?

Cold. Plug makers and Ford's own bulletin say to remove plugs from a cool engine, because heat expands the metal and raises the risk of thread damage. An engine can take a few hours to cool, so overnight is easiest.

Can you put penetrating oil on a stuck spark plug?

Yes. Champion suggests a little penetrating oil to help loosen a tight plug. The key is a small amount at the base of the plug and time to soak, not a well full of fluid.

What happens if a spark plug breaks off in the head?

Part of the plug stays threaded in the cylinder head, sometimes with broken ceramic that can drop into the cylinder. Don't start the engine or try to drill it out. It needs extraction tools made for the job, and for most people that means a shop.

Can you fix stripped spark plug threads without removing the head?

Often, yes. Thread repair insert kits are designed for use with the head on the engine. The work has to be straight and chip-free, so unless you've done thread repair before, leave it to a shop.

Why is my spark plug so hard to remove?

Usually because it was overtightened, it's been in for a very long time, or carbon and corrosion have built up on the threads. Some engines are prone to it, which is why carmakers sometimes publish special removal procedures.

Browse all engine service tools.