How to Replace Spark Plugs & Ignition Coils: A DIY Guide

Summary

• Identify symptoms of failing plugs/coils like rough idling, poor fuel economy, and misfires.
• Gather the correct tools for the job, especially a torque wrench to prevent costly mistakes.
• Follow a methodical step-by-step process, working on one cylinder at a time to avoid errors.

Ever get that feeling your car isn't quite itself? Maybe it’s hesitating when you put your foot down, drinking more petrol than it used to, or starting with a bit of a grumble. It’s easy to jump to complicated, expensive conclusions. But often, the culprit is a small, inexpensive part that’s just had its day: the spark plug.

Changing spark plugs, and their partners-in-crime, the ignition coils, is one of those jobs that sounds intimidating but is surprisingly straightforward. It's a classic bit of DIY car maintenance that can save you a bundle on garage bills and genuinely make your car feel healthier.

So, grab a coffee, and let's walk through it. This isn't about being a master mechanic; it's about understanding your car a little better and having the confidence to get your hands dirty.

"Is My Car Trying to Tell Me Something?" - Symptoms of Failure

Before you start ordering parts, let's figure out if this is even the problem. Your car has its own way of communicating, and when it comes to ignition issues, it's usually not very subtle.

Telltale Signs of Bad Spark Plugs

Think of a spark plug as the match that lights the fire in your engine. If that match is damp or worn, you won't get a good flame. This leads to:

• Rough Idling: Does your car shake or sound jittery when you're stopped at a traffic light? That could be a cylinder not firing properly because of a weak spark.
• Sluggish Acceleration: If you press the accelerator and the car feels lazy or hesitant, it might be struggling to burn the fuel efficiently.
• Poor Fuel Economy: Are you suddenly best friends with the attendant at your local petrol station? A bad spark leads to incomplete combustion, which means you're literally wasting fuel out of your exhaust pipe.
• Trouble Starting: A weak, inconsistent spark makes it hard for the engine to roar to life, especially on a cold morning.
• Engine Misfires: This feels like a stutter or a jerk from the engine, especially when you're accelerating. It's the most direct sign that a cylinder isn't doing its job.
• The Dreaded Check Engine Light: A consistent misfire will almost always trigger the dashboard warning light.

And What About the Ignition Coils?

If the spark plug is the match, the ignition coil is the hand that strikes it. Its job is to turn your car's 12-volt battery power into the thousands of volts needed to create a spark. When a coil starts to fail, the symptoms are very similar to a bad plug, because the end result is the same: no spark, or a weak one.

You’ll often see:

• Engine misfires (sometimes more pronounced than with just a bad plug)
• Frequent stalling or difficulty starting
• A noticeable drop in power
• A strong smell of unburnt petrol from the exhaust
• Backfiring (a loud pop or bang from the exhaust)

A quick but important note on that Check Engine Light: If it's on and steady, get it checked out when you can. If it's flashing, pull over as soon as it's safe. A flashing light usually means a severe misfire is happening, dumping unburnt fuel into your exhaust system, which can quickly destroy your expensive catalytic converter.

Gearing Up: The Tools You'll Actually Need

You don't need a professional workshop, but a few specific tools will make this job a hundred times easier and safer.

The Main Event: A Step-by-Step Guide

Right, let's get down to it. We'll tackle this as a combined job, as you need to remove the coil to get to the plug anyway. Remember the golden rule: work on one cylinder at a time. This prevents you from mixing up coils and creating a headache you don't need.

Step 1: Prep and Safety

• Make sure the engine is completely cool. Seriously. Don't even think about starting if you've just been for a drive. A hot engine can lead to nasty burns and, worse, can cause the threads in the engine block to strip when you remove the plugs. Wait a few hours.
• Park on a flat, level surface and put the handbrake on.
• Disconnect the negative (-) terminal on your car battery. This is an extra safety step to prevent any electrical mishaps.

Step 2: Locate Your Targets

Pop the bonnet. You're looking for the ignition coils, which sit on top of the engine. On most 4-cylinder cars, they'll be in a neat row. On V6 or V8 engines, you'll have a bank on each side. They are usually held down by a single small bolt and have an electrical connector plugged into them. The spark plug lives directly underneath each coil.

You might need to remove a plastic "vanity" cover from the engine to see them.

Step 3: Remove the First Coil

• Disconnect the electrical plug: There's usually a small tab you need to press or lift to release the connector. Be gentle; old plastic can be brittle.
• Unbolt the coil: Use your ratchet to remove the small bolt holding the coil in place. Keep it somewhere safe.
• Lift the coil out: Gently twist the coil back and forth to break the seal, then pull it straight up and out.

Step 4: The Clean-Up

Now you can see the top of the spark plug sitting in its little well. Before you go any further, use your compressed air or vacuum to blow out any dust, leaves, or grime from around the plug. You do not want any of that junk falling into your engine's cylinder.

Step 5: Out With the Old Plug

• Fit your spark plug socket and extension onto your ratchet.
• Lower it into the well and fit it over the spark plug.
• Turn it anti-clockwise. You'll feel a bit of resistance and then a satisfying 'crack' as it breaks loose.
• Unscrew it the rest of the way. The rubber insert in your socket should hold onto the plug so you can lift it straight out.

Step 6: Prepare the New Spark Plug

• Check the gap: Use your gapping tool to confirm the distance between the two little metal electrodes on the end of the new plug matches your car's specifications (check your manual).
• Apply anti-seize: Put a tiny smear of anti-seize compound on the threads of the new plug. Less is more here. Make sure you don't get any on the electrode tip.

Step 7: In With the New Plug

This is a delicate operation.

• Hand-tighten first: Place the new plug into the socket and carefully lower it into the engine. Screw it in by hand. Your fingertips are your best guide here. It should turn smoothly. If you feel any resistance, stop immediately, back it out, and try again. Forcing it will cross-thread the engine block, and that's a very, very bad day.
• The Torque Wrench Moment: Once it's finger-tight, attach your torque wrench (set to the manufacturer's spec) and tighten it. You'll feel it get snug, and then the wrench will give a distinct 'click'. Stop at the click. That's it. You're done. Don't be tempted to give it "one more for luck."

Step 8: Install the New Coil

• Take your new ignition coil and apply a small dab of dielectric grease to the inside of the rubber boot at the end.
• Slide it down over the new spark plug until you feel it seat firmly.
• Put the hold-down bolt back in and snug it up.
• Reconnect the electrical connector. You should hear it click into place.

Step 9: Repeat and Finish Up

That's one down! Now, just repeat steps 3 through 8 for the remaining cylinders, one at a time.

Once you're done, put any engine covers back on, reconnect your battery terminal, and close the bonnet.

Step 10: The Moment of Truth

Get in the car and start the engine. It should fire up smoothly and idle without any of the old shaking or sputtering. Take it for a gentle test drive to make sure everything feels right.

Common Questions & Quick Fixes

• What if a plug is stuck and won't come out?
  Stop. Don't force it. Apply a little penetrating oil around the base and let it soak for a while. Sometimes, slightly warming the engine (not hot!) can help, but this is risky for a first-timer. If it still won't budge, it's time to call a professional before you snap it off in the engine block.
• Do I have to replace all the coils at once?
  If one has failed, the others might not be far behind as they've all done the same work. It's good practice to replace them all as a set to ensure consistent performance, but if budget is tight, you can just replace the faulty one.
• Can't I just clean my old spark plugs?
  On very old classic cars, maybe. On modern cars, no. The special coatings and precise electrode designs are easily damaged. For the price of new plugs, it's never worth the risk.
• I mixed up the coils! What do I do?
  Don't panic. A quick search online for your car's "firing order" will show you which coil connects to which cylinder. This is why we do it one at a time!

Congratulations! You've just completed a key part of your essential car maintenance checklist and given your engine a new lease of life. Now you can enjoy smoother starts, better performance, and the quiet satisfaction of a job well done.

Read our other articles: